If you plan on painting the airplane yourself (priming internal parts doesn't count), then think big (e.g. high CFMs, 240V, 60Gals, etc.). If not, then IMHO you can get away with a more reasonable sized/priced mid-range compressor (see below).
You will get opinions on this that are all over the place, but all I can do is to point out two compressors that have served many builder's well. One is from
Sears (or OSH in CA), the other is available at
Home Depot. I have the Home Depot one, and I have no complaints whatsoever. Both of these are similar in tank size, both are oil-lubed and both are convertible between 120V and 240V. I like the vertical format of the Home Depot one better, but the CFMs on the Craftsman are rated higher.
Just my $.02... YMMV. Good luck!
EDIT: The Home Depot link won't work. But I am sure you can find it.... it is the Husky 5Hp 26 Gal Vertical Compressor, Model VT6315, $377.