I think that 6 gallon one in the Lowes link is about right for an air nailer. If you might have to carry it up stairs, you really want something portable.
Mine is a tiny, light-weight 1 gallon; the smallest one they sell at Home Depot. I wouldn't use it to run an air nailer, but it gets all my household chores done -- filling tires and blowing dust out of tools and electronics. Its only feature is that it is tiny and light-weight!
I was concerned about moisture in the compressed air also, but I tried it on a few things and found the small amount of water evaporates quickly. I've been cleaning computers and audio amplifiers with it for a couple of years and haven't had any trouble.now using a compressor and blow tool to blow dust out of electronics is a bad idea.. the tank collects moisture which then goes through the line and out the blow tool. you should be using canned dry air sold in the computer section or electronics department