The traditional cockatrice was never depicted as being very big, it never needed to be.
Had the mini been on a 20x20 and looked the same as below it would be ok.
A google search on 'cockatrice miniature' reveals several manufacturers making one, mostly they are goblin sized, though some are larger, some smaller and sold in pairs.
Going back to the myths basilisk and cockatrice are largely interchangable.
In fairness thoughthe old Marauder cockatrice model was on a 40x40 base (the largest plastic base of the time) and was broadly similar to many monster models of the time, including the Marauder griffon, and wyvern. So with the scale creep a Warhammer cockatrice could well be a big monster.