I print a lot of snapshots from 6x6cm as 5x5". These have a really smooth look and work well in albums or as giveaways.
35mm I tend to enlarge to around 5.5x8" on 8x10" paper.
6x6 I tend to enlarge to around 8x8" or 10x10", and I do about the same with 6x7 (which I don't shoot often).
4x5" I usually enlarge to 8x10" or 11x14".
8x10" and 11x14" I contact print.
Occasionally I'll enlarge to 16x20" or larger if I think the image works that way, but I usually prefer the tonality of enlargements no more than 4x.