Probably not much more than 23 inch. Here's a couple of links, one for iiyama, the other gives an idea of prices.
http://www.iiyama.com/gb_en/products/touchscreens/#
http://www.novatech.co.uk/search.html?s=touchscreen&c=1097,&m=0
I think they are quoted as being for W7 (of course) so you would need to check for W8 compatability.
Sorry, those links don't copy properly. Add the hash to one URL and monitor to the other. (hopefully OK now)
