It is very simple. If you are leaving the actual fonts alone, when you go to convert your non-drm books to ePub, or ePub to ePub, simply use the font size input and change it to say a base of 11.5. Do the conversion, sync your reader, and view your results. You will find that different fonts look better set at different base sizes, according to your preference. Test the different base inputs from, say 9 to 14 and see what gives you the best looking Small and Medium and Large font sizes on your reader. Kovid's amazing Calibre is very fast and very fun to use. There are many threads on how to change the actual fonts using CSS, and they all look better set at different sizes.
I want to be your LatexSalesMan!