You could match on it with something along the lines of:

    function avoid_too_long_or_too_short()
    prefix set boundry;
    {
        boundry = [ ::/0{8,48} ];
        if net ~ boundry then return true;
        return false
    }

And later on use:

    if !(avoid_too_long_or_too_short()) then reject;

Kind regards,