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It is commonly stated on that a turtle needs 10 gallons of water for each inch of turtle. Therefore, a 6" turtle would need a 60-gallon tank. My question is, is that 60 gallons of water?
My current turtle habitat which is currently a 55 gallon aquarium filled with about 30 gallons of water for a 3" turtle. The remaining volume is basking area/Paludarium.
I've been looking at the Reptizoo brand 64 gallon "turtle tank". if you fill the water to the recommended 6" mark, a 48"x18"x6" volume of water is about 22.5 gallons. However, the base of the reptizoo 64 gallon turtle tank has more water surface area (looking top down) than a 55-gallon aquarium, giving my turtle more area to roam. By the rule of thumb, 22 gallons of water is adequate for up to a 2.2-inch turtle. The other drawback is a have a Fluval FX2, which I feel would be too much flow for 22 gallons of water. It's also worth mentioning my turtle is a female and I'm making preparations for a 10–12-inch turtle (120 gallon?) Any thoughts on water vs total volumes?

If anyone can suggest a turtle tank/paludarium that is bigger that the reptizoo 64, I'm considering all options. I'd like to give My turtle, the nicest possible home. All suggestions are welcome.

Ideally it would be 60 gallons of water, but do the best you can. The ATBA "above tank basking area" is pretty popular to maximize the amount of water in a tank.

The Reptizoo 64 gallon tank you reference is a terraium. Terraiums are not designed to hold much water and are way over priced. For a single female RES, I would look at a 90 gallon aquarium. They are a common size and more widely available, including stands. Although it is a bit narrower, premade ATBAs should fit as well.

There other options as well, from custom tanks to stock tanks.