I wear a size 8 Burton Moto boots, fit me we'll, very comfortable, btw, I ride flow bindings right now too, broke my leg near the ankle several years ago and the Flows allowed me to get back ino the sport without pain.
Ok first off, I'd like to say that being 51 years old and snowboarding is a major achievement! Wow! Hopefully I'll be able to board when I'm in my 50s. (I'm still a teen)
Anyway... If you are looking for a cruiser board and not an "end your life by jumping off a 5 story ramp" kind of board, I suggest a banana-shaped board with the base of the nose and tail higher than the middle of the board. This camber will have more forgivingness than most other boards. It will also make carving easier. As far as the length and width, there's some rules of thumb. The board's length should be within your shoulders and the top of your head. I like my boards at chin level because I'm 5'9" and 138 lbs. the heavier you are for your height, the longer of a board you will want. Also, the longer the board, the more stable and smooth. If you want a really smooth, nice ride, and you plan on having a good time, get a board that is around eye level. For a size 11 shoe, your board should be a medium to wide width. Determine this by setting your boots in your bindings and then on the board you are interested in. If the bindings stick out more than one inch on both sides, you might want to try a wider board. If the boots hardly hang over on the sides, take it down a notch.
If a board is too narrow, your boots will drag in the snow if you carve hard. If it is too wide, you won't have as much leverage when cornering and will make it harder to carve.
I hope I could help solve some of your questions. Keep riding!