What is considered a reasonable length for a math research paper?
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.