I have several ViAir pumps that I keep inside my vehicles and motorcycle trailer. Great pumps for topping off tires, though you have to increase the fuse rating in your car to 15amps, whereas most 12v
ports in your car max out at 5-7amps. Otherwise, buy the version with battery clips or install a dedicated line from the battery.
Since I also wanted a compressor to handle blow-out chores, I did a fair bit of research before landing at the Makita Mac2400. It's only 4.5 gallons, but is oil filled with a great compressor engine, unlike the cheaper porter cable pancake units. The tanks refill in under 2 min and the unit is relatively quiet. I used it with good results to blow out the lines this weekend on my 28", and while it would empty the tanks and recharge frequently, the motor kept up fine and I got past the "mist" mentioned in the earlier post to just air. The cheaper units would take more time to recharge the tanks and the motors might heat up more, with risks of failure, but not the Makita. Plus, the size of the unit is much better in my garage than a 20 gallon tank unit.
If you can live with a larger tank, by all means get the largest one you can for the reasons mentioned. But don't shy away from a smaller unit, if you are willing to pay for the quality. The Makita runs $300 from Home Depot, and another unit by Rolair is also similarly priced and well regarded. Be aware that you may have damage if the units are shipped to you: the first one was dented upon delivery, and I had to get a replacement shipped, which seems to be related to poor packing by Makita. But the second was delivered quickly without defect, and I was able to return the first to the local HD store.
I also used the Makita to blow out sprinker lines, and the smaller tanks quickly discharged. Here, a larger tank would be preferable. However, due to the quick recharge, the Makita completed the task, albeit it ran almost constantly.
For other uses, like airing tires, it is perfect, and I now no longer use my ViAir for adding air to the tires; now it's just a back up for on the-road use. If you fancy running air tools and the like, you will probably be better off going with a larger tank, at least 10-15 gallons.
Some have mentioned that the porter cable and Harbor Freight knock-offs do fine for infrequent winterizing, but I have my doubts about their longevity. My personal ethos is not to cheap out on tools, but you may feel differently.