Except, "realistically," it wouldn't be. Stone is one of those awkward building materials where it's actually easier to work with the larger the material, up to a point exceeding the optimal working sizes of wood or metal.
If you try to make a wall out of two inch pebbles it will take you just shy of forever and you are going to go insane, especially when a stiff breeze cracks it in half (ok, so it wouldn't be that bad, but thin stone has the tensile strength of a saltine).
A wall made of forearm-length, or slightly shorter, stones will require far less time to put together, less mortar to hold together, stack easier, and be far sturdier.
I don't see any reason why stone shouldn't occupy the top slot, unless you're working with plate steel walls as opposed to sheet metal like we have now.
you should know when i say realistic i'm literally talking about realistically making a game. not whether or not you want exact replicas of real life physicality inside of a video game.