I've been looking around and it seems to be just on the top end of what
a keyhole deck will fit, and right on the bottom end of big deck size
(82x50cms, 32.5x20", measurements from outside of cockpit rim to
outside of cockpit rim).

The problem is different manufacturers list slightly different
dimensions for keyhole/bigdeck. I will probably have to buy online as
there are no good shops near me so I would like to get it right first
time....

I'm worried I won't be able to get a keyhole on (no upper body
strength, lol), and I'm worried that a Bigdeck will pop if I get dumped
on.

Any advice on what size I should go for?