It depends upon what you're using it for, the cfm required at the tool and the length of the pipe and hose.
For my 27cfm compressor, I've run a 1" PEX and 1-1/4" black pipe infrastructure across the two shops that the compressor supplies.
The 11cfm compressor on the welding trailer simply uses 3/8" ID air hose.
If you're feeding a sand blaster or air die grinder from a high CFM compressor, then go at least 3/4". On the other hand, if you're only 10 cfm and a small tank (<40 gallons), then 1/2" ID should be fine.