"What is the largest a/c that the pax apron can accommodate?"
If by 'pax apron' you mean the Central Apron - Code 'D', which, although including larger types, normally equates to B767-300 or A300 for this apron.
Two B747 stands have existed for many years on the eastern Apron, which, while originally intended for freighters prior to the western apron being built, could be used for pax aircraft if necessary (subject to the stands not being obstructed by a/c parked on adjacent stands).