Necessity. Though, actually this stuff was painted a long while back. A dozen or so cards were released at GenCon, so I've been posting 'em one a day. In a crunch, though, I can produce them about that fast.
I try to paint at a decent print resolution whenever I work. So for these at 300dpi they're about 14x14". I do have some custom brushes, but it really comes down to making quick decisions and knowing what you want the piece to look like.