As a consolation, at least you can pick to sit in-between two thin people. If you board early with business select, you have no control over how large a person is that sits next to you.

If you are flying a 737-700 (143 paxs) B-50 is probably 60% you will have a window or aisle in the back. If you are flying a 737-800 (175 paxs) or Max, your odds are highly likely you will have a window or aisle. Check to see what version plane you are flying.