I was 160 lbs at 5'6" and wearing a 10/12.  Never wore old Navy sizes, but they probably would have put me at an 8 or something, I have no idea. Most of my stuff comes from the thrift store too. That's a good point about the pre-vanity sizing. My friend is getting progressively fatter, and she claims to be a size 8... and I look at my old fat clothes from when I was a size 8 (once I put my mind to losing weight it took me less than a year to get from my biggest size to a size 0/2 where I am now- I went down a size every couple of weeks)  and I simply cannot picture her in them.  It's one way I'm going to subtly shitlord her- is "Oh you wear size 8? I have a bunch of size 8 stuff from when I was bigger, here, you can have them :) "