It depends on the negative and the viewing distance. The negative, of course, depends on a lot of things - subject, film, camera settings, whether a tripod was used, processing, etc. 11X14 (roughly 5X) will work with just about anything from a 6X7, and 16X20 (8X) will usually be OK. With an exceptional negative, 20X24 (10X) will stand close inspection, and mural size enlargements will work for mural type applications.

To get technical, good eyes will resolve about 300 lines per inch at a viewing distance of 15 inches. Most MF equipment will give you a bit over 2000 lines per inch at the negative. A 7X enlargement from an MF negative is as sharp as your eye can see at a normal viewing distance. These things scale. If you move back to 30 inches, you can have a 14X enlargement with no loss in visual quality. But not all subjects need that kind of resolution. Large block subjects or soft subjects may work with much less, and therefore can stand more enlargement.