That depends on a number of things: film speed, film type, format, how many mega-pixels, did you use a tripod, etc... But mostly it comes down to personal opinion.
A general rule of thumb is that you can enlarge up to 10 times the original before image quality begins to seriously deteriorate. So if you are starting with a 35mm neg that's a 10"x15" print. With a 4x5 neg thats a 40"x50" print.
For my personal work I like to keep 35mm prints at 8"x12" or smaller, although in the past I've made 16"x20" prints from Tmax 100 that I think most people would find acceptable.