Help needed, folks!

Does anyone know the size limits for posting behind an ordinary LJ cut?  Number of words/characters etc??
Part 4 is getting a bit long, and I don't want to split it up before I find a home for a deserving porn scene.
I failed to post Endangered Species as a single fic, so I know that was too big, but I've failed to work out where the tipping point is. Help!