As someone who has recently started collecting posters this is one I'd buy. I collect Shep Fairey (the guy who did the famous Obama Hope poster), Tyler Stout, Dave Kinsey, Joshua Budich, etc to name a few and many of these are in the 18 X 24 format. I use a portfolio case for these but the 18 X 24 format allows you to buy an inexpensive frame without getting a custom frame. As someone who has most of the Obama campaign prints I can tell you oddball sizes make it very expensive to hang these up on the wall.
In any case, you'll want to make limited editions of several sizes and make sure they are signed and numbered. These you can obviously charge more for than standard unsigned and un-numbered prints. Quality offset or digital printing can make a difference too.
My 02 cents.