Good Afternoon, Arcturus,

Obviously, there's a lot of subjectivity involved here. For example, I'm not a fan of grain, so that no doubt plays into my opinion.

After around forty-five years of printing experience, I have my own rough standards. Everything following assumes the use of high-quality camera lenses, a tripod when appropriate, competent, consistent film processing, high-quality enlarger lenses, and the use of modern ASA 100 T-grain film (T-Max 100 being my personal preference):

35mm--up to 5 x 7 for superb quality, 8 x 10 for excellent quality, 11 x 14 for very good quality
4.6 X 6--up to 8 x 10 for superb quality, 11 x 14 for excellent quality, 16 x 20 for very good quality
6 x 7--up to 11 x 14 for superb quality, 16 x 20 for excellent quality
4 x 5--up to 16 x 20 (and probably beyond) for superb quality

Going well beyond 16 x 20 with 4 x 5 negatives would be no problem, except that I rarely have need for anything larger, and the processing gets awkward. Even T-Max 400 negatives should look terrific at 16 x 20 or larger.