It's not a matter that you have too much rock, it's that the pieces you have are too large. You can either take them back to the LFS
if that's where you got them or simpley break them into smaller pieces if returning/exchanging is not an option. Use a small rock chisel and a small weight sledge hammer or a large ball peen hammer. With delicate enough hits in the same area, you can effectively split the rocks into more managable pieces.
Be sure you have someone around to help/supervise, wear thick leather work gloves and protective eye goggles. Wouldn't hurt to do this in a shallow plastic bin outdoors