Practice sleeping in a sleeping bag that is zipped up to only about knee to thigh length to see if you might enjoy a quilt.
I own the 6' 6" Katabatic Palisade, and am 6' 4". It is my first and only quilt. I have used it in nighttime temps from 60F to 35F in the Yosemite Nat'l Forest.
I would not recommend buying a 6'6" unless you've spoken with Aaron Martray (owner Katabatic) because I do not think you'll fit. If you do get a quilt, and sleep in your clothes and use a bivy, I'd suggest you consider the Chisos model as well. These can be discussed with Aaron, though. The bivy and clothes can supplement the 10 degree loss in range that the Chisos covers.
These quilts are excellent quality, and service has been long lauded here, along with several other quilt manufacturers who are listed in gear reviews. But these quilts are not entry-level priced and you will likely be paying extra for long, wide or more custom options. Please practice with with an opened sleeping bag first to see if you are interested in one.