Frags: 141 characters and under.
(Since a fragment is nothing like a blog and forum (doesn’t create a discussion, ect), and it has the potential to eventually be something similar to a tweet, 141 characters is the standard, and should stay that way. If the tweeters can say what they feel in less than 141 characters, then we can too.)
Forums: 141 characters+
(I actually changed my mind about this. Forums should also be as short as they want, if they want discussion and such what. Really, fragments have nothing in common with forum and blogs.)
Blogs: 100 words+.
There you go!
~Mip
Hm. I think threads should have a really low limit since all it takes is a small question to stir up discussion, and threads are allowed to be that short. Anything less than a small question (a small statement or a couple of words) should go as a Fragment however.