I'm sort of at the other end of the scale at an 18/20. I'm also not short at 5 6". And I can shop pretty much anywhere. Maybe not in Miss Sixty or Top Shop but that's fair enough. I think if you're anything from a 12 to a 18 all shops cater for you. Anything smaller or bigger and it's not so easy.
I used to be bigger and that was tough at times. T shirts and basics were relatively easy but quality classic clothes were a nightmare.
As far as QVC goes I do think they cater for larger sizes but some if their ranges are useless for anyone over a 20. I can wear the top sizes in things like Kim n Co (dresses) and Tiana B but anyone bigger than me is going to struggle. Nina Leonard caters for larger than a 20 as does Antthony and probably Susan Graver and Attutudes for Renee.
I've no idea about petites but I'm guessing very few if any would suit. The only one that I would guess might is Michele Hope. Her tops are always short in the sieves and length it seems to me but that's a bit like Hobson's choice isn't it??