Definitely remember the drop when looking! You can't put boot back that isn't there! Look online and you'll find that most tall boots have a sizing chart. Measure yourself according to the instructions, add the recommended number of inches, and see what you can find that fits!
I got my tall boots when in high school (I was 15 I think), and the boots did not drop nearly as much as expected so they ended up being too tall. I just sucked it up through my high school showing days, and the boots have been mostly sitting around while in undergrad. I pulled them out recently to ride in occasionally because I'm contemplating showing again, and I had forgotten how bad the knee crease was! I took them to the local shoe repair guy, and he did a great job cutting them down to the proper height for only $30. You can notice a couple of slight differences when you look closely, but the overall appearance of the boot is exactly the same.