The size of the tank and the power of the pump will determine the output of air compressor. So the very first thing to do when detemining what tank and what power of pump you require is to think about what tools you will be using:

1) Will you be use just one tool at a time? Or would two or more people be using 2 or more tools each at the same time?

2) Tools use air at a rate of CFM. So you need to work out which tool uses the most CFM and ensure your compressor can deliver that figure and a bit more!

There are two types of CFM figure that the tool spec should show:

CFM (Displacement) is the theortical amount of air the compressor will produce in theory. 
CFM (FAD) is the actual value that the compressor will delvier in practice. 

YOU NEED THE CFM (FAD). THAT IS THE IMPORTANT FIGURE.