Limit number of characters for user name

I need to figure out a way to constrain user names to 10-15 characters as for some reason I have a number of people who feel that sentences make great user names :P

I assume this is done in Access Rules and I found a post that made it mandatory to have exactly 9 characters by creating a rule with 9 underscores, but how do I make it cap the characters at say 15 but allow user names consisting of fewer characters as well?

I would laso like to make it impossible to have a single character username.

Any help would be greatly appreciated!

Thanks in advance