CFM matters most
CFM measures air volume. Nailers use air in short bursts, while grinders, sanders and other continuous-use tools can demand much more sustained airflow. Compare the tool's CFM requirement with the compressor's rated CFM at the required PSI—not just the horsepower number on the box.
PSI is pressure, not capacity
A compressor may advertise a high maximum PSI but still lack enough airflow for a demanding tool. Both numbers matter: the compressor must reach the required pressure and maintain adequate CFM there.
What tank size changes
Small tanks are portable and work well for inflation and intermittent fastening. Larger tanks provide a bigger air reserve and reduce how frequently the motor must recover during sustained work. A larger tank cannot compensate for a pump that produces too little CFM.
Garage buying checklist
List the tools you actually plan to use. Check their CFM at operating PSI. Add reasonable airflow headroom, then decide how much portability and tank capacity you want. Also consider noise, electrical requirements, duty cycle, drain access and whether replacement parts are readily available.
Bottom line
For most garage buyers, sizing by the most demanding tool is more reliable than shopping by tank gallons or horsepower alone. Buy enough airflow for the work you expect to do, with some margin so the compressor is not constantly operating at its limit.