The link joraan gave you provided the dimension of each tank just click on the 20lb link, but 20 lbs is heavy unless you drive to get it refill or swap, get a 10lb otherwise. You do save some money on refill on a bigger tank. If you want to see it, go to the location where you plan to refill or swap. It is just more expensive.
derp, didn't notice that lol. I'm sure that wherever I go to refill it I'd have to drive, so weight isn't that seemingly terrible of a thing, especially if I can go longer inbetween refills.
Question as well..........do you ever need to get a new co2 tank or are they prety much good for years and years?