I do think she looks a bit small for you. Not in a your to heavy way at all as you are obviously lovely and slim but my honest opinion is you look unbalanced in a height to height ratio.
Saying that she will fill out a lot of course. My mum looks a little big on her new horse, she's 5ft9 and he's 16.1 and I have to say it has caused them some issues with getting saddles fitted ect. She needs a 17.5 but he really needs a 17inch.
If you find one you like as much which is bigger I would do that. If your sure she's the one and you are really comfortable then it's not like you will hurt her or anything .
I think the mare will look much better for you when she's filled out and muscled up, she's quite immature looking for 5 (compared to some 5 yr olds that is) but that's not a bad thing.
You're not too big for her, nor do you look silly on her , your just tall (and long in your body not just long legs) so I do think you'd suit something much bigger if (IF) you wanted to look 'perfect' that is, but who does, so unless your into your showing big time then I doubt it would even matter.
She looks very sweet, I can imagine her in a few years time when shes in her prime looking like a completely different horse, can just see her sporting some Western tack actually!