You can't win on Mumsnet when it comes to weight and size.
There was a thread last year abut a woman who is a size 14 and said she didn't feel fat, although her BMI was in the overweight range... and she was utterly flamed for it!
Some people on her thread were more thoughtful and helpful - and you know what? Who has the right to care about anyone else's shape or size? - but a lot of posters laid into her saying anything over a size 10/ 12 is fat.
It was utterly awful to watch and, at the time, I was a size 16-ish, having slimmed down from a size 20 (18 in Next/ Tesco/ Asda - see my point below) so made me and many others on the thread feel quite shit. So imagine how the OP must have felt...
But this is very, very true:
I kidded myself for years that I wasn't fat... I fit into a Next size 18 so I was just "curvy"
No, at 170cm (5'7"-ish?) I weighed 111kg and had a BMI of 38.4. That's not just curvy. It was fat.
I'm still overweight by about 11kg but can fit into a Next size 12. Wrong. I have a Moschino dress which is a size 16 and fits perfectly. In other stores I am a 14.
So, I guess what I'm trying to say is that clothes sizing is not a good indicator of weight/ healthiness/ whatever. Weight charts/ BMI etc are also not perfect but they are better than clothes sizing. As someone above said, you can be 6' and a size 18 and slim, or 5' and a size 18 and not slim.
Anyhow, in answer to the original question, most High Street shops go up to a size 18 and some a 20 or more. Next, Gap, M&S etc. all ahve good choices. H&M sizing was weird - sometimes I could wear an 18 but their size 18 jeans wouldn't even go past mid-thigh.