If I remember correctly, the 1650EX is a "hybrid" trailer with two "slide out, canvas walled bed platforms, one on each end. In most Keystone "conventional" models, the bed size is either a standard queen (60x80) or an RV queen (60x75) but with the "canvas wall, slide out bed" models, there are a number of "unique sized mattresses. I've seen models with a standard 54" bed on one end and a king 72" bed on the other end or even one model with a twin bed on one end and a queen on the other. Mattress length can range from "canvas wall to canvas wall" or can be any custom length to accommodate a cabinet or shelf, etc. So, I'd recommend you actually measure the bed to be sure of what's really there.
2015 F250 6.7l 4x4
2014 Cougar X Lite 27RKS