From Ariel Rubinstein of course, here’s his answer to question 5:
Q5. I have already written 30 pages. I have repeated myself several times and my proofs are much longer than necessary. I have added uncertainty wherever I could and I have moved from a discrete case to Banach spaces. My adviser still says I hardly even have enough for a note. How long should my paper be?
If you don’t have a good idea, then keep going. Don’t stop at less than 60 single-spaced pages. Nobody will read your paper in any case so at least you have a chance to publish the paper in QJE or Econometrica.
If you have a really good idea, my advice is to limit yourself to 15 double-spaced pages. I have not seen any paper in Economics which deserved more than that and yours is no exception. It is true that papers in Economics are long, but then almost all of then are deathly boring. Who can read a 50-page Econometica paper and remain sane? So make your contribution to the world by writing short papers — focus on new ideas, shorten proofs to the bare minimum (yes, that is possible!), avoid stupid extensions and write elegantly!