Foam thickness is mostly a matter of personal preference, but in general, you don't want to go any thinner than 2". It also depends on how you intend to use your bench cushion; if it's a window seat for lounging, maybe go with 4" or 5", or if it's a kitchen table bench, 2" or 3" should be fine. I chose 3" foam because it's a good balance of cushy and soft feeling, and not outrageously expensive.