Pressure needs vary, just like with people of other sizes.
For arms that may not fit on the table, rather than having them tuck their hands under their own hips (which as a bigger person I can tell you is more work for the client), I use the drape. Except when working the back, you can actually just tuck the drap right around the whole arm/hand and anchor it firmly under the hip. It's a little "coccoony," but if the client likes it it really holds arms gently and securely.
As far as supporting the arm while working on it, you can sit on a stool and put a pillow in your lap. This essentially makes your lap an extension of the table, and the client will feel well-supported.
I am another huge fan of double-articulated facecradles, which can come up high above the table surface while still keeping the neck in a good therapeutic position. If the cradle is up quite high when the client is face-down, I like to put "shoulder pads" under each anterior clavicle area -- basically just a thick, compactly-folded flannel pillowcase that keeps the shoulder from rounding sharply forward/downward.
I would probably not ask a client to move around on the table for working on the arms -- I feel like it's my job to pamper them, and my responsibility to make the equipment work for their comfort. Maybe the school has a pair of side extensions you can borrow to attach to your table for this session? I have a pair permanently installed on mine and all clients, large to small, love the feeling of security of the wider surface.
One thing you have in your favor here -- this is a massage instructor so he is already accustomed to making his own adjustments for comfort -- although he will appreciate the effort you make. Enjoy doing the massage without necessarily mentioning that you are making special accommodations -- it's great practice for the kind of adjustments you'll be making for clients all the time when you're in practice!