there are charts around for cable sizes vs amps vs length of cable.  they're pretty accurate.  you also have to look at how many amps you're gonna use.  if you're just gonna be stick welding with no more than 200 amps in the shop then #1 is about all you need.  if you're gonna use all 400 amps and you're 200 ft away from the machine, I think 2/0 or 3/0 would be the way to go.  i'm just guessing though. 

in fact, every new cable connector, stinger, and ground clamp has a cable size chart stamped on the box.  drop by your welding supplier and they'll set you straight.

O/2 welding cable will work fine for all welding purpouses, unless you are doing a lot of heavy work there shouln't be much need to be heavyer. A whip of #1 cable is nice for 10 - 15 ft for stick welding so you don't have to hold all the weight, I also have one heavyer for the big stuff. cable size is #4 #3 #2 #1 - 0/1 0/2 0/3 0/4.  #4small   0/4big.