I shoot large plates for wet plate collodion. As Jeremy mentioned all the way up to 20 x 24. When the big plates are done well they really shine beautifully. For much of my wet plate work I settled on 10 x 12 as my size of choice. I like the bridge that it gaps between what I found for my eyes to be the too small format (8 x 10) and the 11 x 14 which just didn't work for me for whatever reason. 10 x 12 can still be handheld (fun with your wet plate ambrotypes) or have a nice weight to them on the wall. If you buy the 11 x 14 you might want to make a plexi insert and play around with the 10 x 12 size. Remember if you are buying a camera that you plan to use a conventional holder that is modified you will not be able to use the 'full' size of the film holder for wet plate as you will have downsize some. Of course none of that will come into play if you buy a wet plate camera with a wet plate back on it. Pouring the plates won't make that much of a difference between the two. Once you get to practicing with whatever camera you choose you will figure that part out relatively quickly. Wasted collodion is expensive you'll either adapt or get out of the game!
Good luck--wet plate can be addictive. Don't forget to check out Quinn's wet plate forum and of course you can contact me if you wish for any further thoughts or guestions. There are many wonderful wet plate artists on APUG that will have much to offer--Kerik, Bill, Matt, Jersey Vic, Joe so don't be shy about getting started and rolling with it.