Laptop - 1440x960 - (screen very close compared to most other systems - ~0.5m):

System font - Bitstream Vera Sans Mono 13
Screen font - Bitstream Vera Sans Mono 15
Ambient Desktop font - Bitstream Vera Sans 18 (Alpha Shadow White/Black)
Ambient Window font - Bitstream Vera Sans 18 (Alpha Shadow White/Black)

If I remember rightly only a few of the supplied fonts look 'modern' and are suitable for use as both screen-font (any font type) and system-font (fixed width only). This might influence choice somewhat.

Desktop (14" screen, 1280x1024, distance ~1 - 1.5m):

Same as above, except:

System font - DeJaVu Sans Mono 20
Screen font - Bitstream Vera Sans 18

When the mac-mini was used as a HTPC (40" screen, 1920x1080, distance ~3 - 4m), the screen font size was set to around 30 (possibly bold).

PS - For desktop fonts I found readability at a distance and/or over a range of colours/contrasts - at least on predominantly dark backgrounds - was best accomplished using the Alpha Shadow mode (after playing about with glow/shadow/outline fonts).

[ Edited by boot_wb 16.10.2016 - 13:13 ]