With my current site style, 15 characters is causing the username to overflow to another line and my dilemma is whether to slightly redesign and/or limit the username size so that it does not go over.
Update: After analyzing a forum with a little more then 18k users (and never had a username limit), here are the stats for username length of current users.
21-25 characters: ~ 30 Users ( About 10 actually posted at least once)
16-20 characters: ~ 200 Users(About 50 actually posted at least once)
15 characters: ~800 Users
11-14 characters: ~6.5k Users
1-10 characters: ~ 11k Users
With that in mind, there is no need to allow over the 15 char length (in the case of the forum analysed).