"Wait, that didn't work right" - the Dysfunctional Rules Collection

[Vladislav]

That seems fine to me. A big shield throws off your balance, and maybe you'd have to swing that hand around a lot to cast and thus the shield could get in the way sometimes.

JaronK

However, there are no rules for Arcane Spell Failure for holding in your hand any large and bulky item which is not a shield. You can cast spells just fine while holding a dumbbell, or a ladder, or a sack of treasure, for example. A buckler? Heaven forbid, no.