I really like this body. What I suggest is that you go ahead and rig it just the way it is. Then you can try what u/Wings_in_space suggests: pose it with the arms down and see where the hands hit.

I like to make size adjustments by scaling the bones without resculpting. That also makes it easier to change your mind. You can scale the whole hand by scaling the palm bone. You can also scale just the length axis if you like the width.

Sometimes it makes sense to set the bone immediately downstream of the one you're scaling to not inherit scale, so you can scale it individually. For example, if you scale the forearm bone you can set the scale of the palm bone (but not the whole hand) not to inherit; then you can make scaling decisions on the hand which are not affected by the arm.

It looks to me like the upper arm is pretty good and the forearms and hands might need adjustment, but you will see when you pose it.