I'm going to create a snowman 3do for a level I'm working on, but I need to know how detailed I can make it before it hurts the level's framerate.

If the snowman were to use the traditional 3 spheres stacked on top of one another, how many surfaces could each sphere have?

Id say if you were going for a low number not too many. But I dont see Why you would need anymore or anyless than around 200 surfaces if you want detail.

try and make layers, which will turn in to nodes in the 3do. try to have no more than 200 in each layer, but you can have as many layers as you want.... not that a snowman will need that many