The Britax Frontier 85, in booster mode, is the tallest HBB. Excellent leg support...but it's a lot of money to drop for "just" a booster, especially one that does not later convert to backless.
The Britax Parkway SG/L and Cosco Pronto are the tallest dedicated HBBs. The Pronto, however, requires adequate vehicle head support and isn't, generally, as well liked as other HBBs. The seat depth on the PWSG/L isn't fabulous, but my DS doesn't complain. He's also 52 inches, but is more torso then legs. You could place one of those foam coolers on the vehicle floor for her to put her feet on for additional leg support.
My 9-year old has outgrown all HBBs except the FR85 in booster mode and the PWSG/L. He rides in a PWSG in my vehicle and a backless Monterey in DH's.
Many 9-year olds will still fit in a Monterey. I wish mine did. It's just all about how their body is proportioned. So, for a 9 year old, I would definitely try out any HBBs before making a purchase (on the child, in the vehicle, and then the child IN the booster IN the vehicle). Or purchase from a return-friendly store.