[_ck_]

That’s another one of those silly things that got carried over from WordPress. The database limits it to 60 characters and the default template limits it to 30 characters but it’s still overly long.

Fortunately in bbPress (unlike WordPress) the registration form is done in a template and not in the core, so the easiest way is just to modify the register.php template under my-templates (or copy it from bb-templates) and change the line that looks like this:

<td><input name="user_login" type="text" id="user_login" size="30" maxlength="30" value="<?php if (1 != $user_login) echo $user_login; ?>" /></td>

And instead of 30, make it the size limit you want.

However people that are sneaky and bypass the registration form can still make it 60 characters (ie. spammers). But they will stick out, so you’ll notice it right away.

This can be fixed via a plugin but it would be way too much code to justify the fix. The template method is best.