Go for the biggest and most reliable you can afford. Most can be adjusted in output anyway, or you can use T-pieces to supply other reservoirs.
Careful though . . . some of the membrane pumps make a lot of noise.

I've successfully used the little $5 ones for fish tanks for years! They last about 2 or 3 years, then quit and I get another one. As long as the pump is powerful enough to deliver air under whatever head pressure you have in your reservoir (1/2 psi per foot), it's a go!

The fish tank pumps are usually rated according to the number of gallons of water they're designed for. Of course that's for keeping fish healthy, not hydroponics.

I usually double or triple the number to be safe. The only way you can have too much air is if it literally blows the water out of the reservoir.

You have to know the pump head (pressure) to get watts/gallon.

Flow X pressure = Hp = watts