There must be a sizing guide somewhere. You've been given overlarge armpit crutches (I think they only carry one size honestly and they fit about 25% of the people who use them). On the forearm crutches, you care about height of the stick part (adjustable but you are wanting a shorter one to begin with) and watch also the length of the part that runs from below elbow to wrist. Too short and it will strain your wrist, too long and it will be uncomfortable and hook on your clothes and not work.

A lot of US orthos don't use them or don't know how to fit them. You may be on your own there.

On using them---you're pretty fit, a key thing is, use core muscles for stabilizing yourself; drawing down shoulder blades and firing that bra-band muscle set (forget the name) when you swing yourself off stairs or curbs is very helpful. Put the crutches down securely, lean forward onto them, touch down with good foot.

Good luck, you'll be much more comfortable with them.