I'm also very long-legged and have never been able to sit properly at a piano of normal height. My home piano is simply raised about an inch (just enough to allow for knee clearance) by three, 10" diameter steel disks. They're ordinarily used for weightlifting, so conveniently there's a small hole in the center of each disk that prevents any slippage.
Since a grand can weigh half a ton, I suggest getting some help.
My Kawai grand sits 24" at the bottom of the keybed, but I have it raised to 25".
I strongly suspect that there is an industry standard.