Hello and welcome!
I'm not as tall as you are - but my legs are very long...
My first tips for you would be to make sure everything "fits". Make sure your stirrups are adjusted properly and that your horse is a good size for you. If your stirrups are too short or if your horse does not take up a lot of your leg - you're going to feel as though you are towering way over him every time you try to rise in your stirrups.
Are you working with an instructor?
Leasing a spoiled rotten trail horse...pretty - but a brat!