This question comes up a lot on APUG, and you can find many good threads about it. How big an enlargement you can make depends on a lot of things - the quality of the negative, the subject, the viewing distance, the printing technique, how steady the camera was, etc. Traditionally, 35mm was printed to 5X7, and that is still an excellent format for it. But you can almost always get a good 8X10 (roughly 8X) enlargement out of a decent negative. If the camera was on a tripod, and the exposure and development were correct, you can often get a quite decent 11X14. Going bigger is a very iffy thing. Some subjects do not depend on detail, and a bigger enlargement works well. If the print will only be viewed at a distance, or if it is used to set a mood rather than to be viewed critically, bigger enlargements work. How big? It depends on the subject and the use of the print. I've seen many good 16X20 enlargements from 35mm, but I've seen many more terrible ones. For some mood setting photos enlargements up to 6 by 9 feet are reasonable (but not very often).