Don’t think height plays a part, unless you are 6’6" tall.
How heavy is 11 stone?
At 200lbs., you can easily go down to 5’10" x 22 as long as you keep thickness to close to 3". That would float like a normal shortboard around 6’9" long.
Example, Quicksilver’s rep here, at 185 lbs, rides a 5’6" x 22 fish as his small wave board. His next board up is a CI Flyer 6’4"er. His fish floats about the same as my 7’9" Bic, and paddles close, but it catches waves later, of course.
Remember, the board I rode was compared to my Bic7’9"er, a full on fat boy Mal, and it paddled about the same, but caught waves later, of course. Float not much diff, and the fish’s thickness was about 2.75" with a flat deck and boxy rails.
And I was 54 years old at the time, hadn’t surfed in over 9 months.