If this had happened to me I'd have told her 'you've just lost yourself a sale' ... and walked straight out of the shop. Then I'd have gotten on the phone to their head office and asked to speak to the area manager or whoever and told them they'd just lost a customer and why.
How dare she comment on your size? Not just rude but patronising as well "oh I bet your glad we stock the larger sizes in here, I don't expect there's many places you can shop now in the high st". Even though it didn't happen to me I'd be absolutely furious. She's a shop assistant ffs - there to assist, nothing more.
If anyone thinks my reaction is over the top - then all I can say is there's plenty of people who don't have a job out there - she's one of the lucky ones. And with businesses closing left right and centre, they should be doing all they can to help and assist customers and secure sales for their employers, not making rude and patronising remarks.
I was shopping with my son - buying him a pair of shoes, and we went into a well known High Street shoe shop. My son picked out the shoe he wanted off the rack, sat down and tried it on (there were three women behind the counter at the end of the shop and not one of them came over, even though we were the only ones in there) said they fitted and he wanted them, so we'd served ourselves basically .. and I took the shoe up to the counter.
I stood there and was completely ignored. The three women were talking about rotas and time sheets and didn't even look up .. even though I was no more than 2ft away - just the other side of the counter. After standing there for a couple of minutes as if I was invisible I plonked the shoe down on the counter hard (that made them look up lol

), and told them they'd just lost a sale. When I got home I was still wound up about it, so I rang their head office and ended up getting a call back from their area manager who was appalled. I don't know what happened but I hope they got into trouble - they deserved to. Empty shop, three staff, one customer who had served themself and they still couldn't make the sale.
That particular shoe chain is now in big trouble with many shops likely to close, and I can't say as I'm surprised.
We're very good in this country at just putting up with bad service, and downright rudeness, turning the other cheek, whereas in other parts of the world they just wouldn't tolerate it.
Report her. Complain. You've got every right to.