Remind me of your tank size.
If you have a tank over about 50g, then you'll want atleast a 10lb co2 tank IMO. The larger the co2 tank you have the longer it will last between refills. Its hard to guess, but I'd say a 10lb tank would last you close to a year on a 50-60g tank...maybe longer. If you don't mind refilling more often, a 5lb would be ok. Personally I would get atleast a 10lb.
I'm not sure where you were planning on getting yours, and the height may vary, but I think this will give you a pretty good idea:
http://www.aquariumplants.com/product_p/t.htm.
You don't get the co2 into the tank....you take it to a welding shop or restaurant supply place or a fire extinguisher refill place and they put the co2 into the tank.
I suggest you look around at places to fill the tank before buying. You may only have places around that will swap tanks out instead of refilling on site. In that case, I suggest buying there and then trading in when you run out. That way you don't buy a brand new tank and end up trading it in.
I bought my tank from Aquariumplants.com. I looked at prices locally first and got a 20lb tank from AP.com for the same price I would have paid for a 10lb tank here. Then I took it to a carbonics place and they filled it for me. The price to fill it varies by location. IMO $8-10 for a 5lb tank is a pretty good price.....even better on a 10lb.