If you don’t know what you like and don’t have the ability to try others, then take a quality boot sock and fill it with Polly Fill beads. You can also use a quality dress sock, as they have a bit more stretch but they are thinner (obviously).
Poly Fill beads can be found at any arts & Crafts store for less than $10. They are better than airsoft BB’s because they are smaller and will allow for fine micro adjustments. Best environment-proof option for a fill that you can find anywhere.
Play with the fill level until you find the size that works best for your shooting style. Then you can compare that size to that of the commercially available bags and go from there.
Honestly you can’t beat a personal “sand sock” as a dedicated rear bag for range days. Lightweight, custom-fitted/made to fit you, and just plain personal.
Hope this helps.
Also, your firing position/target angle will obviously dictate the size bag. You could have 12” bipods but not have room for a bag, just as you could have a 6” bipod and need more than a Solo Sack on end for support. Situation dictates (and the elements of a proper firing position should be present) but the great thing about “sand socks” is you can stack em and making two, or even four, is cheaper than pretty much any single rear bag out there. ...especially if you don’t know what you like or need.