most of the ones we made for the care facility were between 30" x 40" & 36" x 45"...those seemed to fit most everyone---sometimes we had a request for a shorter one (30" x 30") and for a couple pretty large guys- in big-motorized chairs we made ones that were 36"x 54"...
best if you know the person is to measure- across the seat of the chair (usually about 24") add a couple inches to each side to tuck it in (over/around them) then measure from waist to ankles- with them sitting- you don't want it too long- or it can tangle with foot pedals or catch on wheels.