I use Bitstream Vera Sans Mono/13 for system default text and Bitstream Vera Sans/15 for screen text on the Mac mini with a screen resolution of 1280x720. For Icons I use Bitstream Vera Sans/15 and /11.
I have reinstalled the OS quite recently on the PowerBook G4 (HD died...) and I found the default setting good enough not to mess with them - default screen on the 17" should be 1680x1050.

Kind regards,
Andrea