I think I'm pretty much the same height and slightly heavier than you. I just took delivery of a 14.2hh middle weight cob.
I think it depends on whether your long in the leg, the body or both.
Stig takes up my leg but my body looks kinda silly.
I'm the same height as you (but two and a 1/2 stone bigger!) and the samllest I rode comfortably was a 14.1 Arab (I was 11 1/2- 12 stone at the time) but I like the size Hattie is, 15.3. not too big, not too small!
I'm 5ft 3 and I make my 14.2hh look really big, lol... you'd be absolutely fine on him and he's still young so still has a lot of filling out to do, which will mean he'll take up my leg even more and I'll look like a pea on a mountain, lol (I have very short legs)
The two horses in your siggy? You look about right on them, so wouldn't recommend anything smaller than 15hh/ 14.2hh chunky chap... You won't weight them down, but you will look very long!