Many short papers or one big paper for a math postdoc? As the title says, I am a math postdoc. I am working on a project, which I expect will result in a long paper (60-100+ pages, I am not really sure yet). As I work along the way, I am getting results, which are probably interesting on their own.

I wonder if it would look bad on my resume if I will have many short papers (say, of less then 10-15 pages)? or having many papers at this stage of the career is good for me? 

Should I balance between these two approaches? What are you thoughts on this, assuming I am interested in getting a tenure track position at a research university?
This really depends on the situation, and I suggest you consult with some colleagues/mentors/your advisor for your specific case.  Here are a few points to keep in mind.


If these results are completely separate, then it makes absolute sense to break things up into separate parts, then assemble the various needed results for the endgame of your project.
If you're just continually improving results, it may not be worth while to publish every little improvement, particularly if they will not go into good journals, but you might want to look for natural breaks.  
One great paper is better than many mediocre papers, but at this stage you should equate a few good papers with one great paper.  
There's no general rule for how much you should publish, but a good rule of thumb is to aim to have at least decent 1 paper/year since you graduate.  Keep in mind, that papers always take longer than expected, and the refereeing time is generally long in math (particularly more so for longer papers), so the sooner you can submit, the better.


So you should take into account the rest of your publication record, a conservative estimate of how long the project will take, and what quality of journals you can publish in if you break things up in to parts versus keeping it as a whole.  If you can break things up and still publish in good journals, then I would say this is the safer way to make you a good candidate for tenure-track/second postdoc positions.

Edit: I just realized I didn't directly address the OP's second paragraph.  It's not bad to have many papers at this stage if those are good papers (though it can initially look suspicuous if you have many more papers than your peers, and most don't appear in good journals).  In some fields, it is normal to  mostly write many short papers (and for certain subjects, all papers under 15 pages would be strange).  What's most important is that people think you are doing good research, which will come primarily from letters of recommendations and secondarily your list of publications.  So while you should think carefully and seek advice from colleagues on how to publish your papers, but one long paper versus a few short-medium papers will probably not affect your application too much if they're accepted by the time you apply.