How tall is the tank? Sounds like it could be anywhere from 40 to 80 cubic feet. Check the link below, should help you determine its size:
http://www.tricowelding.com/cylinder_sizing.html
The biggest factor in determining tank size, especially regarding oxy/acetylene setups, is figuring out how much usage they will really see. If you are truly going to use them once or twice a week, then you need to get the largest size tanks you can afford (or the largest size your LWS will supply). Keep in mind that some shops will lease out larger tanks than the ones they exchange (on customer owned tanks). For example, I own all my own bottles, and my Argon tank is 125 cubic foot. Doesn't matter if I owned a 230-250 cubic foot tank, my LWS will only swap me out with a 125 cubic foot tank. Now, they will lease me a 230+ cubic foot tank of Argon all day long, and will exchange it with the same size all day long, but its not the same with customer owned tanks (and I haven't figured that one out).
As long as the tanks don't need to be too portable, go with a 150 cu.ft sized Acetylene tank and a 230-250 cu.ft tank of oxygen. You can wheel them around on a cart, or you can simply park them in a corner of your shop and pick up a set of long hoses. 50-100' hoses are relatively cheap, and make the tanks a lot more versatile. You can compromise get a midsized set of tanks, say a 150 cu.ft O2 and 70 cu.ft acetylene, and see how they fit your needs. Just know that the larger tanks aren't that much more expensive to buy outright, and the price in exchanging them isn't much different. Having a 150 cu.ft tank of acetylene exchanged isn't much more than a 70 cu.ft tank.
I have a total of 11 tanks that I own outright, with three pairs of oxy/acetylene tanks. I've yet to buy any of them new, and picked all of them up through local deals or Craigslist. My LWS will allow me to "trade-up" tanks, meaning I can trade smaller tanks for larger ones and pay the difference. I have a small set (like the one pictured above) of oxy/acet, and midsized set, and a large set. I use the bigger set the most, as it sits in the shop and I roll out the hose when I need something cut quickly. The medium set is on a hand cart with large rubber wheels, and makes rolling around outside much easier. The small set is simply for when I need to take the tanks somewhere else for a quick cut.
Honestly, though, I use my plasma cutter far more often than the torch. That being said, the oxy/acet setup is far more versatile.