I use Miyuki (sp)11/0 and most of the time I can get 22 and 24 gauge wire to work. If I use other seed beads which I rarely do, then 'not so much' as the holes are not the same. and neither are the sizes.
If you use 6/0 and 8/0 seed beads, then you could use 20 gauge wire easily, just remember to bend loosely as the seed bead will break if you put them in a bind.
15/0 seed beads 26 and 28 gauge wire is probably the largest you might be able to use.
Hope this helps.