Log in as an administrator and check your forum settings, it's in there (where exactly I forget, but it is, I promise!) I think it defaults to 5000 characters...
It's worth mentioning that the FAQ that Stash cited is about discussion topic lengths and not comment length. Comment length can be edited in the Forum Application Settings page.
Comments
Ah yes...
P.S. Mark/Wallphone/etc it looks like the formatting in the FAQ is borked (and I don't have the time/inclination to fix it...)
It's worth mentioning that the FAQ that Stash cited is about discussion topic lengths and not comment length. Comment length can be edited in the Forum Application Settings page.