The size and margins of the pages a FlowDocument creates: the media box is [0 0 Width Height] and the
content area is the media box less the margins. Immutable; With* returns a changed copy.
public PageSetup(double Width, double Height, PageMargins Margins)
The page width in points.
The page height in points.
The margins around the content area.
ISO 32000-2 §7.7.3.3, Table 31 (MediaBox) and §14.11.2 (page boundaries).