What size air hose would a mechanic's garage require?
You need to understand the pressure figures you are referring to. What you are probably referencing is the pressure rating of the reel units, this is the pressure they can operate at safely (without the hose bursting or some other unfortunate dangerous failure).

The air delivery capacity will be expressed by a quantity per unit of time, like "liters per minute" or "cubic feet per minute". Generally, larger hose can deliver more air and some types of tools need a lot of air (like rotary air tools). Can you find out what pressure he operates his unit at? If I were you I would default to the old idea that "bigger is better", or just ask him which one he would like. Most men very much appreciate nice tools for gifts, whether or not it is a suprise.