A general rule of thumb for blast nozzle sizing is the air compressor needs to be able to provide 50% - 100% more CFM than the CFM requirements of the nozzle. New blast nozzles wear rapidly and require more CFM the more they wear. At 100 psi a new 1/4" sandblast nozzle only needs an 18hp compressor that provides 81CFM at 100psi. After some time that 1/4" nozzle will wear to 5/16" then to 3/8" and larger. The nozzle will then flow more CFM and require a larger compressor. Looking at the chart 5/16" requires 30.5hp to provide 137CFM at 100psi. 3/8" requires 44hp to provide 196CFM at 100 psi. If you have a 30hp compressor you can use a 1/4" nozzle until it wears to 5/16" then it must be replaced. If you have a 44hp compressor you can use a 1/4" nozzle until it wears to 3/8". Nozzles are expensive. The 44 hp compressor would require fewer nozzle changes than a 30 hp compressor in this example.