What is considered a reasonable length for a math research paper? I've read that many mathematics journals have page limits. What are they, roughly? I think in one of Steven Krantz's pieces he says that if a journal is going to publish a 50-page paper, then "it had better glow in the dark." A friend of mine heard that more junior faculty (straight out of grad school) should avoid publishing papers more than 30 pages long.
Different math journals specialize in papers of "around" a certain length. Trans AMS, for example, publishes long papers. The web sites give the limits and they are, I assume, pretty rigidly enforced. 

I don't know that such long papers need to glow in the dark, but they need to justify their length. It is possible that a single theorem has an exceeding complex and long proof, the details of which are mostly new. If the theorem is important, then it will be justified. 

At the other end of the scale a paper with somewhat more modest results, but which presents a complete (and novel) body of knowledge that might be extended by others, likewise warrants a long paper. 

But don't try to make a paper extra long just to try to get into Trans AMS. It won't be helpful. And don't make a paper short by leaving out essentials making it impossible to understand. 

As short as possible, but no shorter, is probably the right idea.