I do mine every year with an old Sears compressor, maybe 10 gal
capacity, delivers 6.4CFM@40 I added a gate valve and male air fitting
on the water supply side.

I let it get up to about 70PSI, then turn on the zone. I go through
all the zones twice.

It's true that more capacity will do it faster and probably get a bit
more water out. However, I've been doing it 8 years and can't
attribute any problems to freezing. And I've saved $500.