your hubby will be about average in Florida - the Americans are generally a bigger race. don't worry, he'll fit although some thingts like dumbo might be a bit of a squeeze.
Try this link on allearsnet and they have pictures and descriptions of every ride seating (its more geared towards 'Pooh' sized people, i.e severly overweight) but it may help for a general overview. http://allearsnet.com/tp/ridsiz.htm
Also as for a bad back I can only think that Big Thunder, Splash and Space mountains in MK might be a problem, Everest and Dinosaur in AK, ToT and RnRC in MGM and Test Track in Epcot.
others will add if they've found any specific difficulties.
thanks a lot. I am mainly interested in water rides anyway so hopefully will get enough enjoyment out of the parks despite having to just watch everyone else. Cant wait to get there now 33 days to go and counting