On the Registration page (/ucp.php?mode=register), the following text is used to describe a user's "Username":

Length must be between # characters and # characters.

Using the word "characters" twice in the same sentence is not needed. It is unnecessary redundancy.

Shortening it would use up one less row of text, making it look cleaner while still getting the point across.

I suggest the following instead:

Length must be between # and # characters.