I am looking to buy a Caldwell Lead Sled DFT shooting rest. It suggests to place four 25lb bags of lead to hold the rest down and to absorb the recoil. They say lead is the best material to absorb the most recoil, sand is next best choice.

What size lead pellets should I buy to absorb the most recoil?

Fine No. 9, larger BB or doesn't matter as long as the weight is there. Thinking of getting 2 bags of No. 8 and 2 bags of No. 4.