Full screen: 720x576 32 (50Hz) PAL
Windowed: 640x512 or 720x576
-- 05 Lip 2013, 21:28 --
its hard to exacly replicate amiga resolution, cuz of various modern monitors we have (you should get an old amiga crt monitor or connect your pc to old school crt tv, its a best option). these settings are really close to real look (for better "aspect ratio" view try use bilineral filtering, but it'll be lil bit blury)
here you have real amiga 500 resolutions:
PAL resolution:
320 x 256 (from 2 to 4096 colours),
320 x 512 (from 2 to 4096 colours),
640 x 256 (from 2 to 16 colours),
640 x 512 (from 2 to 16 colours)
those are spec's of european amiga system. but there were other resolutions too:
overscan resolution 768x564,
NTSC 320x200, 320x400, 640x200, 640x400
so as u can see there's a lot of options

hope i helped
peace