[Logician]

For site admin there should already be no restriction. For mods edit your private.php and find this:

-- cut --
if (strlen($message)>$pmmaxchars and $pmmaxchars!=0 and $bbuserinfo[usergroupid] != 6) {
-- cut ---

Replace it as

-- cut --
if (strlen($message)>$pmmaxchars and $pmmaxchars!=0 and $bbuserinfo[usergroupid] != 6 AND $bbuserinfo[usergroupid] != X) {
-- cut --
Replace x with mods group id..

Enjoy..