You're kind of asking for a really broad spectrum of anime by putting the upper limit at 50 episodes. Most anime only run 26 (two seasons), and that's considered a lot sometimes.
I have no problem with how long a show is, I'd rather they tell the story well. For example, I loved the first half of Claymore (the show, not the manga), before they get to the north. After that the story just went to shit. It would have been a lot better had they not dragged it out like that (among other things). On the flip side, though, I absolutely loved Requiem for a Phantom, fit snugly into 26 episodes.
In terms of 'could a story hold up another season', I think a story should be written in full at the time of conception. Tacking something on only works sometimes, because it seems like most writers lose the feel of the story and the atmosphere once they've finished production and then decide to go back and try again.
Your view of short would really only bar a few anime (such as Blood+, Bleach, Naruto, One Piece off the top of my head) as usually they clock in at less than thirty episodes.
