WordPress really needs an option to limit the length of user names during the registration process.
This should be a simple numeric field entry in the General Settings.
The number of WordPress installations that use forums and allow other user comments is huge. Long user names always break the design layout of these sites.
From the research I've done, I can tell you this is an issue that has some history, and has been sorely neglected by the dev team. This should be a core feature, not a plugin solution (not that there's any plugins which address this issue).
Thanks for considering implementing this feature in the next update to WordPress :-)