My printing sizes vary considerably - actually it makes more sense to talk in terms of enlargement factors - 17x is about the upper limit I can manage with my enlarger and 35mm, using a 50mm lens, and I do print that big sometimes. Between 8x and 12x is more usual for me though. With larger formats obviously the factor comes down somewhat - 6-8x maybe for medium format, and 4x or 5x at the most for 5x4.

However, grain is rarely, if ever, the the thing that determines how large I print a neg. I have no rooted objections to its appearance, so I decide on the basis of other things, like the impact of the print, or whether it can hold up in terms of sharpeness and detail, and that sort of thing. And ultimately it's the size of print that I can handle - 20" x 16" at the most in my darkroom - that stops me going any bigger.