air-compressor size for continuous duty
Your tool and your compressor both have (or should have, but they probably do) ratings in "Standard Cubic Feet Per Minute" (SCFM) possibly stated just as CFM.

The compressor SCFM needs to be equal to or greater than that of the tool to operate the tool continuously. 

Small "homeowner-grade" compressors are often not rated for continuous operation at all (they will overheat if asked to supply their maximum SCFM for more than 10 or 20 minutes at a time.) Industrial compressors are more typically designed to operate continuously for long periods without failure.