I found out the answer to my question by doing some experimenting. Once you have the texture image in your editor, you can re-size it, and it will still apply properly. I had thought of this a while back, but always assumed the oversized texture wouldn't fit properly.
I re-sized the King Air's main texture to 2048x2048, painted it blue, and added nose art to try it out. It worked, but I forgot to float the prop spinners to my "details" layer, so they got painted over. Now when I try to fix it, Dxtbmp isn't overwriting the file. Anyone know why this would happen? Here's what it looks like, can't get rid of the blue on the spinners. The girl's hiding behind the engine because she isn't wearing anything. XD