i think it really depends.
long anime series (those who probably have a thousand or so episodes) can be good, given that a lot of character development happens/can happen. but at the same time, they tend to be dragging and/or boring if the creators stick to the same plotline through the entire series. take naruto, for example. i've known lots of friends who used to be big fans of naruto, but now, they claim that they're getting sick of it since it's too long (i'm not quite sure if the series is finished already though).
on the other hand, short anime series, if the plotline is good enough, can leave you wanting more. no matter if a conclusion has already been drawn; because of the fewer episodes it has, the audience would probably want to see more of the characters and/or some plot development.
but the relationship of the quality of the anime and its length really is relative.