I shoot film because it has grain (and other properties that are unique to the medium). I like the beautiful tones of tri-X and i love the fact as you get closer the grain emerges, adding another layer to the viewing experience. A couple years ago I shot the same subject with tri-x in 6x6 and 4x5. As I recall I enlarged the 6x6 to 16x16 it was still nice, but getting grainy the 4x5 was still silky @20x24 (or it may have even been larger my memory is not so good).
From a proper viewing distance a good picture will still look good large. I think a better question to ask is, "What type of picture should be enlarged to X big?"
Not only am I less bothered by grain than others here, but hold affection for it. Grain is part of the medium. I really am somewhat confused that some feel that a 4x5 is maxing out at 8x10. I am not as well versed in the black and white arts as others here, but I'm pretty sure that the resolving power of paper is still no match for the detail held in a 4x5 neg at an enlargement of 8x10.
I enlarge colour negs from 6x6 to 20x20 square all the time. It is apparent by what you can see in the grain focus that there is still detail to be had -- detail that is not being picked up by the paper. And yes, grain is visible (not nearly to the degree found in B&W). the nature of colour film (three layers) minimizes grain somewhat.