您的问题对于什么是“最佳”矩形不太清楚。我假设您的意思是打印时 100% 可见的最大矩形。
让我们首先确保我们了解打印文档图形对象的“原点”是什么以及 OriginAtMargins 属性如何影响该原点。
OriginAtMargins - 获取或设置一个值，指示位置是否
与页面关联的图形对象位于
用户指定的边距或可打印区域的左上角
页面。
- PrintDocument Class Definition on MSDN
所以OriginAtMargins 设置为false（默认）图形对象将被调整为 PrintableArea 矩形（对于我的激光打印机，每个页边大约 5/32，旧激光打印机可能更多，新喷墨打印机可能打印到边缘，软件 PDF 打印机将打印到边缘）。所以我的图形对象中的 0,0 实际上是我的激光打印机物理页面上的 16,16（您的打印机可能不同）。
在默认的 1 英寸页边距和 OriginAtMargins 设置为 true 的情况下，图形对象将调整为 100,100,650,1100 矩形，用于正常的纵向字母页面。这是每个物理页面边缘内一英寸。所以图形对象中的 0,0 实际上是物理页面上的 100,100。
边距也称为“软边距”，因为它们是在软件中定义的，不受物理打印设备的影响。这意味着它们将应用于软件中的当前页面大小，并反映实际页面尺寸纵向或横向。
PrintableArea 也称为“硬边距”，它反映了您的打印设备的物理限制。这将因打印机而异，因制造商而异。因为这些是硬件测量，所以当您将页面设置为横向/纵向时，它们不会旋转。无论软件打印设置如何，打印机的物理限制都不会改变，因此我们需要确保根据打印文档的软件设置（方向）将它们应用到正确的轴上。
因此，按照您发布的示例代码的粗略模型，这里有一个 PrintDocument.PrintPage 事件处理程序，它将绘制一个尽可能大的矩形，同时仍然可见（默认 PrintDocument.OriginsAtMargins 为 false）。如果您将 PrintDocument.OriginsAtMargins 设置为 true，它将绘制一个尽可能大的矩形，同时在配置的软边距内仍然可见（默认为距页面边缘 1 英寸）。
PrintAction printAction = PrintAction.PrintToFile;
private void printDocument_BeginPrint(object sender, PrintEventArgs e)
{
// Save our print action so we know if we are printing
// a preview or a real document.
printAction = e.PrintAction;
// Set some preferences, our method should print a box with any
// combination of these properties being true/false.
printDocument.OriginAtMargins = false; //true = soft margins, false = hard margins
printDocument.DefaultPageSettings.Landscape = false;
}
private void printDocument_PrintPage(object sender, PrintPageEventArgs e)
{
Graphics g = e.Graphics;
// If you set printDocumet.OriginAtMargins to 'false' this event
// will print the largest rectangle your printer is physically
// capable of. This is often 1/8" - 1/4" from each page edge.
// ----------
// If you set printDocument.OriginAtMargins to 'false' this event
// will print the largest rectangle permitted by the currently
// configured page margins. By default the page margins are
// usually 1" from each page edge but can be configured by the end
// user or overridden in your code.
// (ex: printDocument.DefaultPageSettings.Margins)
// Grab a copy of our "soft margins" (configured printer settings)
// Defaults to 1 inch margins, but could be configured otherwise by
// the end user. You can also specify some default page margins in
// your printDocument.DefaultPageSetting properties.
RectangleF marginBounds = e.MarginBounds;
// Grab a copy of our "hard margins" (printer's capabilities)
// This varies between printer models. Software printers like
// CutePDF will have no "physical limitations" and so will return
// the full page size 850,1100 for a letter page size.
RectangleF printableArea = e.PageSettings.PrintableArea;
// If we are print to a print preview control, the origin won't have
// been automatically adjusted for the printer's physical limitations.
// So let's adjust the origin for preview to reflect the printer's
// hard margins.
if (printAction == PrintAction.PrintToPreview)
g.TranslateTransform(printableArea.X, printableArea.Y);
// Are we using soft margins or hard margins? Lets grab the correct
// width/height from either the soft/hard margin rectangles. The
// hard margins are usually a little wider than the soft margins.
// ----------
// Note: Margins are automatically applied to the rotated page size
// when the page is set to landscape, but physical hard margins are
// not (the printer is not physically rotating any mechanics inside,
// the paper still travels through the printer the same way. So we
// rotate in software for landscape)
int availableWidth = (int)Math.Floor(printDocument.OriginAtMargins
? marginBounds.Width
: (e.PageSettings.Landscape
? printableArea.Height
: printableArea.Width));
int availableHeight = (int)Math.Floor(printDocument.OriginAtMargins
? marginBounds.Height
: (e.PageSettings.Landscape
? printableArea.Width
: printableArea.Height));
// Draw our rectangle which will either be the soft margin rectangle
// or the hard margin (printer capabilities) rectangle.
// ----------
// Note: we adjust the width and height minus one as it is a zero,
// zero based co-ordinates system. This will put the rectangle just
// inside the available width and height.
g.DrawRectangle(Pens.Red, 0, 0, availableWidth - 1, availableHeight - 1);
}
确定可用宽度和可用高度的两条线是我认为您在问题中寻找的。这两行考虑了您需要软边距还是硬边距，以及打印文档是配置为横向还是纵向。
我使用Math.Floor() 来轻松删除小数点后的任何内容（例如：817.96 -> 817），以确保可用宽度和高度刚好在可用尺寸之内。我在这里“不安全”，如果您愿意，您可以维护基于浮点数的坐标（而不是 int），只需注意会导致裁剪图形的舍入错误（如果将 817.96 舍入到 818然后打印机驱动程序决定它不再可见）。
我在戴尔 3115CN、三星 SCX-4x28 和 CutePDF 软件打印机上以硬边距和软边距在纵向和横向上测试了此过程。如果这不能充分解决您的问题，请考虑修改您的问题以阐明“魔术矩形”和“最佳矩形”。
编辑：关于“软边距”的注释
软边距应用于软件，不考虑打印机的硬件限制。这是有意和设计的。如果需要，您可以在可打印区域之外设置软边距，并且输出可能会被打印机驱动程序裁剪。如果这不适合您的应用程序，则需要调整程序代码中的边距。您可以阻止用户选择可打印区域之外的边距（或在他们这样做时警告他们），或者您可以在实际开始打印（绘制）文档时在代码中强制执行一些最小/最大条件。
示例：如果您在 Microsoft Word 2007 中将页边距设置为 0,0,0,0，则会弹出一个警告对话框，显示“一个或多个页边距设置在页面。选择“修复”按钮以增加适当的边距。”如果单击修复，Word 将简单地将硬边距复制到软边距，因此对话框现在显示所有边距为 0.16"（我的激光打印机的功能）。
这是预期的行为。如果打印页面被剪裁，这不是 Microsoft Word 的错误/问题，因为用户忽略了此警告并使用了 0,0,0,0 页边距。在您的应用程序中也是如此。您需要对用例中适当的任何内容实施限制。使用警告对话框，或者您可以在代码中更强烈地强制限制（不要向用户提供选择）。
另类策略
好吧，也许您不想只获得硬边距，而是获得软边距，然后在打印时强制软边距保持在可打印区域内。让我们在这里制定另一个策略。
在本例中，我将在边距处使用原点，并允许用户选择他们想要的任何边距，但我将在代码中强制选择的边距不在可打印区域之外。如果选定的边距在可打印区域之外，我只需将它们调整到可打印区域内。
PrintAction printAction = PrintAction.PrintToFile;
private void printDocument_BeginPrint(object sender, PrintEventArgs e)
{
// Save our print action so we know if we are printing
// a preview or a real document.
printAction = e.PrintAction;
// We ALWAYS want true here, as we will implement the
// margin limitations later in code.
printDocument.OriginAtMargins = true;
// Set some preferences, our method should print a box with any
// combination of these properties being true/false.
printDocument.DefaultPageSettings.Landscape = false;
printDocument.DefaultPageSettings.Margins.Top = 100;
printDocument.DefaultPageSettings.Margins.Left = 0;
printDocument.DefaultPageSettings.Margins.Right = 50;
printDocument.DefaultPageSettings.Margins.Bottom = 0;
}
private void printDocument_PrintPage(object sender, PrintPageEventArgs e)
{
Graphics g = e.Graphics;
// If you set printDocumet.OriginAtMargins to 'false' this event
// will print the largest rectangle your printer is physically
// capable of. This is often 1/8" - 1/4" from each page edge.
// ----------
// If you set printDocument.OriginAtMargins to 'false' this event
// will print the largest rectangle permitted by the currently
// configured page margins. By default the page margins are
// usually 1" from each page edge but can be configured by the end
// user or overridden in your code.
// (ex: printDocument.DefaultPageSettings.Margins)
// Grab a copy of our "hard margins" (printer's capabilities)
// This varies between printer models. Software printers like
// CutePDF will have no "physical limitations" and so will return
// the full page size 850,1100 for a letter page size.
RectangleF printableArea = e.PageSettings.PrintableArea;
RectangleF realPrintableArea = new RectangleF(
(e.PageSettings.Landscape ? printableArea.Y : printableArea.X),
(e.PageSettings.Landscape ? printableArea.X : printableArea.Y),
(e.PageSettings.Landscape ? printableArea.Height : printableArea.Width),
(e.PageSettings.Landscape ? printableArea.Width : printableArea.Height)
);
// If we are printing to a print preview control, the origin won't have
// been automatically adjusted for the printer's physical limitations.
// So let's adjust the origin for preview to reflect the printer's
// hard margins.
// ----------
// Otherwise if we really are printing, just use the soft margins.
g.TranslateTransform(
((printAction == PrintAction.PrintToPreview)
? realPrintableArea.X : 0) - e.MarginBounds.X,
((printAction == PrintAction.PrintToPreview)
? realPrintableArea.Y : 0) - e.MarginBounds.Y
);
// Draw the printable area rectangle in PURPLE
Rectangle printedPrintableArea = Rectangle.Truncate(realPrintableArea);
printedPrintableArea.Width--;
printedPrintableArea.Height--;
g.DrawRectangle(Pens.Purple, printedPrintableArea);
// Grab a copy of our "soft margins" (configured printer settings)
// Defaults to 1 inch margins, but could be configured otherwise by
// the end user. You can also specify some default page margins in
// your printDocument.DefaultPageSetting properties.
RectangleF marginBounds = e.MarginBounds;
// This intersects the desired margins with the printable area rectangle.
// If the margins go outside the printable area on any edge, it will be
// brought in to the appropriate printable area.
marginBounds.Intersect(realPrintableArea);
// Draw the margin rectangle in RED
Rectangle printedMarginArea = Rectangle.Truncate(marginBounds);
printedMarginArea.Width--;
printedMarginArea.Height--;
g.DrawRectangle(Pens.Red, printedMarginArea);
}