My
largest Fell is 13.2hh. I'm 5'5". As said, they 'take up the leg' as they are so wide. And they are so chunky, an adult doesn't look at all amiss atop 'em.
I love Fells - when they do spook at something it's only a wee skip to the side, then a good look and maybe a snort. None of this histrionic stuff you can get with the 'leggy horses'!

Even when Dave was really frightened of a muck spreader which still had its chains going, he just pranced on the spot 'cos my hands were telling him not to go forward.

I only ever think about his height if we are out on a ride with 'leggy horses' and I find myself looking up to talk to other riders! But most people around here have natives - Fells, Dales, Highlands and one Exmoor - so we're in good company.
And most of the natives I know are good honest forward-going creatures. One or two may be a bit inclined to buck-kick now and again - and most will take exception to anything but very occasional, and must be deserved, use of the whip - but on the whole they are interesting and enjoyable to ride, and capable of working hunter level competitive work. The conformation of the jaw and neck means they find higher-level dressage a challenge; they simply can't get their heads pulled in that far without pain, but if you don't mind them carrying their heads comfortably, many are willing. (And some just think dressage is silly and would rather go for a proper ride

)
I've ridden a couple of cobs in the past, and I think most of my comments about natives would apply to them too (although they perhaps were less likely to have their own opinions and the intellect to make those feelings known!) Whether all cobs are temperamentally similar to natives, I don't know - but if you are enjoying riding this mare, and she's already yours... I'd stick with that for a while and see how far you can go with her. Once you're out and about with her, you'll come across others doing similar, someone will know of a nice one, etc...