You can change the Base Font Size on the Layout page, but it only accepts down to the tenth of a point. Sometimes just tweaking it a little bit one way or another makes a significant improvement in the overall layout of a book, especially for longer books.
I believe that this Base Font Size is just converted to the corresponding \FontSizeUnit, where 1.000pt corresponds to 12 pt. So if you want more precise font size, you can add these lines to your
ptxprint-mods.tex file (the first two of which are comments):
% For more precise font size, divide into 12 % E.g. 10.85 / 12 -> 0.904pt \FontSizeUnit=0.904pt
Then do the calculation to get whatever precision font size you want. This will just override whatever \FontSizeUnit gets defined from the Layout page.