I usually print my own posters, but I've got a commission to do a design only and the band's manager hasn't sourced a printer yet, so I'm free to do it to any size I want. I personally normally print on A2 paper, but as whoever ends up printing it will be US based, I figured it would be the right thing to do to work to a standard US poster size - if such a thing exists.
I heard a couple of mentions of 18" x 24". Would that be about right?
Yeah, that's pretty common. But there's two sets of standards. Printers' standards and framing standards. For printing, parent sheets in america are usually 26x40. A pure half sheet is 20x26, so I use that size a lot myself. But paper vendors always build some loss into their products, and if you're printing a bleed you'll cut down anyway. Thus the 18x24's, 12x16's, and other bastard sizes of the world.