A few years ago, I helped our scout troop build some kayaks with nearly the same lumber dimensions. I don't recall a single issue with the cross sections being too small. I'll let others chime in on what works for a folder...
As to the size of the boat, there are lots of reasons to paddle a canoe and not all of them are about speed. If you are happy with the size, don't worry about it. For example, smaller boats are easier to maneuver in tight quarters and can go places you can't reach in a large boat. It just needs to do what you want.
Steve