Feed size

I just got a dummeril boa from a girl that couldn't keep it any longer. I have no means of contacting her and I wanted to know general feed size. The snake is 5'7 and about 3-4 inches thick

You want to feed a rodent that is about as wide as the widest part of the snake. (slightly smaller is fine)

Also keep the snake on a lighter feeding schedule than others her size, Dumerils have a somewhat slower metabolism and tend to fatten up easily.