Will this be her main seat or just for occasional use in your vehicle? (i.e. do parents have a w/c van or SN seat?).
If this is a primary seat, I think I'd be starting the process for a special needs seat. You could buy some traditional seats that would fit a kiddo with a 17" torso, but for a child that old, with long-term need for a harness (i.e. she's not going to be booster ready in a year or 2), a seat with some positioning assistance and longer use would probably make more sense. Now, as for specific seats or how they fit? I only have experience with the Britax Traveler Plus, and it's a beast (I guess most seats to fit big kids will be). It's on the Regent Platform. I'm thinking a Special Tomato might be a little narrower, but that's just my guessing. I have no experience there.