Don't worry I figured it out.
For anyone using the 3.6 version of this modification you need to go to:
Admin CP > Styles & Templates > Search In Templates > quick_register
Then change in the quick_register template:
Code:
<input type="text" class="bginput" name="username" maxlength="15" value="" style="width:70px" />
Change the number of that value to whatever your max number of characters for a username are. You have to do this for all the styles you have.