Launching and recovering solo, I find a 9.9 hp 4 stroke is my limit for safe lugging about. I believe a 15 hp 4 stroke is approximately the same weight. That places a limit on every aspect of my SIB. I chose a 310.
I can easily launch and recover a 310 with plywood floor, solo.
I used to have a slightly bigger boat with fatter tubes and could do that but with noticeably more effort.
Some of my happiest SIBing was with a 3 hp 2 stroke. It was no strain at all to carry or store it. OK, so you can't whizz about on the plane all day, but you can explore, chug about, and appreciate the wildlife.
If I was choosing again, I might go for a slightly bigger hull (330?) and a smaller engine (6 hp?) which probably is the opposite of what many of you would choose.
These days, I spend a lot more time in the sailing dinghy, so the SIB only gets one or two river outings a year.