It depends on how long your work sessions are.
The ideal ergonomics for drawing-related activities are to have a large surface - 24 to 36 inches - at a drafting table type angle. You should be seated like at a desk with legs at 90, maybe alternating with standing every hour or two. And then while seated your torso is erect and you’re looking a couple degrees down.
The larger surface is so you can lock your wrist and be pivoting your elbow and shoulder without lifting your arm too much.
The longer your work sessions, the closer to this ergonomic ideal you’ll want to get so you don’t injure yourself.
Obviously 12.9” or 11” are smaller than this, so you should limit your work session length to just a couple hours or something short like that.
But at least you should prop up your iPad on a stand so you’re not hunched over, and if possible have it connected to an external display with the top edge a couple inches above eye height.
For really short sessions, especially when highly mobile, a tablet holder is really helpful. There are ones on amazon with rings that swivel 360. You can put your hand in the ring or even forearm and hold it in different positions.
Generally, think of smaller versions as shorter session devices.