In Calibre you can set the base font size. - Preferences-Conversion-Look and Feel
You can change the font using PRS+ and css sheets. See here:
https://www.mobileread.com/forums/showthread.php?t=64510
The best font is all personal preference. Personally, I like Google's Droid font.
http://damieng.com/blog/2007/11/14/d...oogle-ascender