Is there a way I can limit the number of characters in a member's forum post?
No, there is no way to limit the number of characters in a post in Eve.
Sounds like there's a problem you're trying to solve.
Maybe if you tell us the problem, we can help you come up with a solution.
There is one user who repeatedly posts extremely long posts dominating the forums. I was hoping to limit the length of the user's responses.
This is one of those community manager situations that can be unpleasant to deal with. If this is just one user, your best option is to contact that user and make it clear that you value the input, but that it's too much and maybe stifling the other users. It's tough to find a nice way to say that someone should tone it down, but it's sometimes necessary.