i have a 7 year old with ASD, i have found the following to have good seat sizes, bob revolution- seat very roomy, hood clearance ok, very very easy to push, tfk joggster,seat very roomy, hood height better than the bob, easy to push, but not as easy as the bob, the p&t's but NOT the dot or smart [sport, verve, vibe, explorer] seat huge, hood clearance huge, push easy but harder than the others, getting up kerbs,much harder with the explorer, jump on handle jobby, although foot on axel helps [newer one] verve and vibe kerbs ok, still harder than the bob and tfk, sport is the easiest for kerbs, but bob and tfk much easier.
if you want a pushchair that will last you and your child is already the size of a 5 year old, then the p&t's are prob your best option, but you will be compromising with the push and kerbs, over the bob or tfk. [although saying this, its only the hood clearance that may pose a problem as he gets taller, mine like to curl up in the seats, so i will get away with it for a bit longer, he is also average size, maybe a bit smaller] i also have a mountain buggy SN terrain, it may have a fixed wheel, but it is sooooo easy to push, the seat is deeper on the sn version and the hood height is more, it will take upto a 9 year old, its also very light to lift/carry, they are discontinued now though, so a saved search on ebay would be required, but they do come up.
out of the list up there, i currantly use a tfk joggster, bob, P&t sport, p&t hammerhead and mountain buggy sn terrain