I am a 4th year PhD student, and I am substantially done with a paper (area: Political Economy, Theory) that my advisor thinks is very interesting and would be worthy of publication in a good journal if appropriately extended and revised. However, realistically, the paper would be about 25-30 pages including proofs. My advisor's input is that a really long JMP is not really "journal ready", and my advisor personally prefers shorter, cleaner papers.
On the other hand , it seems like virtually every job market paper is in the 45+ page category. I think a short paper might be good because it might be read more carefully. On the other, I'm worried that the page length itself is a signal, but I'm not really sure of what.