The rule save_paper_figure() uses to pick a type size from a figure width.
Text in a figure should read at roughly the body-text size of the document
the figure is placed in; since a figure is usually placed at 100\
means the type size has to follow the physical width. The rule is calibrated
so that the two standard widths land on sensible values: 3.33 in (a
two-column journal column) gives 7 pt and 7 in (full text width) gives 9 pt,
with linear interpolation between and clamping outside.