The more I shoot medium format, the more I think I need never use 35mm again. Even for a carry around, happy-snapping camera, there are the Fujifilm autofocus 6x4.5 cameras. And there are old 6x6 folders like Zeiss Super Ikonta which make Leicas seem like large cameras.
As for print size, it's all opinion, and of course types of film. Ektar 100 in 35mm has about the same resolution as a 20MP FF DSLR, but in reality, you've got to do some pretty good scanning to get that resolution out of it. Medium format makes it much, much easier, and of course you can use faster/grainier films.
I think you can notice an improvement in even quite small prints, but once it's in a frame, on the wall, and you don't have your eyes 2 inches from it, then I think you can get away with quite a lot.
The way I see it is this, 35mm is capable of getting a lot of resolution, good enough for most people, but it can be a bit tricky sometimes. Medium format makes getting a load of resolution simple. With film, resolution drops off a lot when you get faster film, digital does not drop off so much, and it's more noise/lack of saturation than lack of resolution.
If you want high resolution easily, then digital is a pretty good choice. If you don't mind just a bit more effort, medium format film, for me, is a great balance of very inexpensive cameras and superb results.