4" = 2lb
8" = 10lb

then you order by 33#, 45#, but your machine can only handle up to 10lb spools. If you don't weld for a long period of time (2 weeks), take out the spool and get it into a climate controlled area. This will prevent rust from building up on the wire.

A bottle will only last you as high as you crank up your gas flow. Following proper gas flow usage will extend the life of the bottle. Miller weld calculator has gas flow on it I believe.

Buy the same wire, every wire runs a little different. Not talking thickness, talking manufacturer. This will help you get the handle of it faster. Hobart wire is usually available anywhere (Home Depot, Tractor Supply, ect) and is a good overall wire.