I guess the size would all depend on your style of shooting.
6-9 " Benchrest size is dandy for F class, prone position, or "truck hood" shooting
8-13" is great for prone hunting or prone hunting in higher grass or obstacles
12" - 25" is super for sitting position or very tall grass , or calling coyotes, etc.
All depends on what your application is. I would suggest you network with some folks and try a few heights to discover your comfort level and what works for you.
I've got a 6-9"BR for my Sniper competitions and a 8-12" for my groundhog hunting forays.
Lots and lots of Harris Bipods come up for sale frequently on the EE Forums.
Cheers,
Barney