A few months back I asked Jim Keaton to make me a 12x12 heatbed for my dad (username: REdington) and he did and sent me the prototype. Well He is now selling them online.
He has an 4.5x6, 6x6, 6x8, 6x11, 8x8, 8.4x12.6, 10x10, 12x12, And even an 18x18
Link to his ebay store. He is in the USA so shipping is really fast.
http://www.ebay.com/sch/3dheatbeds/m.html?_nkw=&_armrs=1&_ipg=&_from=The reason I am posting this is because LOTS of people online have been looking for a large heatbed solution and someone finally is making them for us. I can contest to 12x12 as I have used it. It heats up dang quick and get pretty hot when maxed out.