Are fonts set to specific sizes always printed having the requested physical sizes?

No, it depends on the value entered in the Scaling of fonts/markers box in the Documents/Graphs: Settings options pane of the Options dialog (accessible via the Tools menu). If 100 (e.g., 100%) is the value (the default value), the fonts will be printed at their specified physical sizes (as set in points; 1 point = 1/72 of an inch). Specifically, the fonts will appear printed in their exact physical size, i.e.:

This is 4 point

This is 6 point

This is 8 point

This is 10 point

This is 12 point

This is 14 point

This is 16 point

This is 18 point

This is 20 point

The manner in which the font size settings as specified in the graph translate into the actual physical sizes of the fonts that are displayed or printed can be globally adjusted in the Documents/Graphs: Settings options pane of the Options dialog. This can also be done interactively for individual graphs using the Decrease Font  or Increase Font  buttons on the Graph Tools toolbar.