What would I need to do to change the allowable number of characters for the bio field?

In changebasic.php find this

// ------------------------------------------------
// If Bio is greater than 250, the we do not allow
if ( strlen($Bio) > 250) {
$html -> not_right($ubbt_lang['BIO_TOO_LONG'],$Cat);
}

change to something like

// ------------------------------------------------
// If Bio is greater than 250, the we do not allow
if ( strlen($Bio) > 400) {
$html -> not_right($ubbt_lang['BIO_TOO_LONG'],$Cat);
}

Thanks Chuck.

I always thuoght that was configurable in the config.inc file. I've learned something new

I did too, but it appears that only the signature length is configureable.