Lets talk tanks.
Do I want to buy a tank?
Should I rent/lease a tank?
What is a good cubic feet size?
It is much more cost-effective to buy a tank, versus renting. If you can swing the cash, do it. Rental costs may be around $100 / yr or thereabouts, whereas a new tank may be a few hundreds of dollars, so after three or four years, you've bought your cylinder. If you're not sure you're going to be welding that long, maybe rental makes sense, or if you just can't afford the up-front cash of the cylinder, then at least rental will get you welding, but buying makes more financial sense, hands-down. If you do buy a cylinder, make sure you are buying from a reputable seller. If someone sells you a rented cyilnder, you are going to be up a creek when you go try to fill it and they just take it back and keep it. Not sure how you can tell if a cyilnder was rented or not.
It is much cheaper to fill a bigger cylinder, on a per-unit basis. On TSC's web site, for example, 40 c.f. of shielding gas is $0.79 / c.f.; 80 c.f. is $0.52 / c.f.; and 125 c.f. is $0.39 / c.f. That being said, you should not buy such a big cylinder that it becomes unwieldy to move around, assuming portability is one of your needs. A 125 c.f. cylinder is 7" diameter by 43" tall and weighs over 60 lbs. That might be more than you want to be rolling around on your little cart.
A bigger cylinder also requires fewer refills, which means less time spent in the car or truck going to get it topped off.
EDIT TO ADD: Also, if you buy your cylinder, you can get it refilled anywhere, but if you rent it, I believe that you typically can only refill it at the place you rent it from. Not 100% sure on that. I have been researching an acetylene rig this week, which is where my info is coming from. I have not personally owned any compressed gasses yet.