My area that I poured for my lathe was 6" and when running the lathe my starrett machinist level didn't budge.
A lot of your pad needs are based on your use. If you are in a shop with a lot of shock (tires being busted, jacks, hammering...etc) you will have more need for isolation than a single shop operator doing one op at a time.
Mass equals accuracy....but there's diminished returns.
