For the larger person's frame, watch out for the seat style with many of the chairs. I have chubby thighs (there might be a more discreet way of phrasing that) at just under 6' and 18 stone. The AK Racing Premium chair I have has a quite pronounced side bolster to the sides of the seat, a design I see on a number of chairs. That forces you to sit very leg's together which might be good for the posture, but not necessarily relaxing. Don't get me wrong, I like my chair, but there are times when I push back, rather than am gaming that something wider would be nicer.
It's nice to see an increasing range of chairs on sites like OcUK, but this is really something quite difficult to buy online when you really want to be able to park your bum on a few cushions and see which really fits. After all one size doesn't fit all.