It doesn't matter what length the anime is, as long as I like it.

But I usually prefer medium-length animes, like "Kuroshitsuji" and "K-ON!". They're not insanely long like "One Piece" or "Bleach", but I have enough time to spend watching them.
Short-length animes are okay, as long as the story is detailed and fun to watch, like "Angel Beats!". But they're not long enough to keep myself entertained, though.
I can't really say for movies, since I watch them only on special occasions, and that they are definitely NOT a length of 20-40 minutes.
And, as for long-length animes, I just watch one episode from one series everyday. I don't have the time to watch 300 episodes.... >w>