I'm doing okay with a 15KW. Could've got an 18 for only a few hundred more, but after looking at the fuel consumption difference at half load, the smaller one seemed like a better choice.
I may have up to 1,600 gallons in tanks, but could also be down to under 1,000 at times (not counting about 500 in vehicle tanks), so lower fuel consumption could be a good thing.
Of course, I would make sure that both water heaters aren't on before welding, for example. But so far the 15KW has done what it's been asked to do without any issues.