I too drive an e36. Love that car. If only Eurosport twin screws were still available...
I forget the numbers, but my palm circumference was just into the lower end of medium by maybe a cm. The end of a medium hand rest is perfectly shy of my hand width.
Cyro is the freakin best! You will not regret the purchase, but it may take a little tweaking to make it perfect for you.
I'm not certain, but the buttons may be 9mm squares. Have some cabinet bumpers or something similar on hand. The 3/16" thick ones in the pic are 3/8" diameter and they overhang the buttons more than I'd like, but do a fine job of putting awkward buttons in reach. The 1/16" thin ones (smaller -- idk diameter) are just there to protect the buttons. Square bumpers, domed bumpers, or something else may work better for the thick ones and grip tape may be better than the thin ones, but that's what I had available and it works well enough for now.
Also, unless something changed,
these things have an issue with the velcro squares coming off the hand rest. I stumbled on a cure purely by accident and
documented the procedure to help others.
Have a good one!