For impact wrenches, any size will work.

Dont consider anything smaller then 4-5 hp and 60 gallons if you will be doing body work or sand blasting. In fact, Sand blasting requires an 80 or 120 gallon tank. By body work, I mean running sanders and spray guns. These have constant demand needs. An Impact wrench is only on a few seconds at a time.

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1959 Gravely LI, 1963 Gravely L8, 1973 Gravely C12
1982 380SL
1978 450 SEL 6.9 euro restoration at 63% and climbing
1987 300 D
2005 CDI European Delivery
2006 CDI Handed down to daughter
2007 GL CDI. Wifes