There are no RAW rules for oversized shields.
As a house rule, you might allow a large buckler to be used by a medium sized character as a heavy shield, and a large heavy shield to be used as a medium tower shield. A large tower shield would be unwieldable, except perhaps as a semi-mobile wall.
An oversized shield would deal more damage as a weapon, but wouldn't give more AC.
The closest thing to an oversized shield for defensive purposes is probably a Tower Shield.