Get the biggest tank you can....you won't be sorry.
Before I ordered my Powertig 250EX, I had bought from a private party a 125 cubic foot argon tank. It was full and a good deal at $100. 125 cubic foot tanks look kind of big but........it isn't. I highly recommend that if you get a tig welder and do lots of welding, get yourself the 250 cubic foot tanks or larger if available. The 125 lasted me about 1 week and so I checked into trading in the 125 for a 250. The gas dealer said no problem. I just have to pay $125 plus give them my old 125 and the 250 is mine full of argon. Not too bad.
The rumor is that you cannot buy 250 cubic foot tanks and larger any more. Not true. You can buy tanks if they are available from a private owner or a gas dealer that still sells them.
The gas dealers prefer that you lease the tanks as they make some pretty good money off the leases. But if your dealer is a decent dealer, they will sell you a larger tank.
I had to modify my welding cart to hold the 250 but that was easy. What I do like is I get twice the gas at a cheaper rate than if I bought two 125's and had them filled. Next to the 125 I have for the mig (75/25), the 250 looks huge.
So the moral of the story here is get the largest tank you can. As big as a 125 is, it goes quickly when you are welding almost every day. Better to have a 250 or larger.
Powertig 250EX
Powerplasma 50
Hobart Handler 210 with spoolgun
Cobra 2000 / Henrob O/A torch
Drill press / metal brake / 36 ton air hydraulic press
Franzinated modified Craftsman 33 compressor
Lots of other metal working tools