Rules for using undersized shields

[Yogibear41]

I know about the rules for taking negatives to using undersized weapons, and you basically can't wear undersized armor, but are there rules for using undersized (or oversized) shields? What happens when a large sized creature picks up a medium sized heavy wooden shield? What about if said creature straps a medium sized buckler to his arm? Is there a rule for this? Or is it simply "you can't" Would a magic shield still provide other bonuses even if it was the wrong size and unable to give a bonus to AC?

[Anthrowhale]

I know about the rules for taking negatives to using undersized weapons, and you basically can't wear undersized armor, but are there rules for using undersized (or oversized) shields? What happens when a large sized creature picks up a medium sized heavy wooden shield? What about if said creature straps a medium sized buckler to his arm? Is there a rule for this? Or is it simply "you can't" Would a magic shield still provide other bonuses even if it was the wrong size and unable to give a bonus to AC?

I haven't seen any rules about this, so the default answer would be 'no'. In the case of magic armor though, it might adjust across sizes. See here.