A chunk of our spawn area in Vokselia, done as a test. This is the absolute largest (at least in X and Z - the height limit is 7.8 inches high) that you can print in color, it's as large as the ZPrinter 650 printer can go. I was surprised, I thought the cost would be in the thousands, but it's "just" $633.14.
The area here is 190 x 125 blocks, 2 mm wide. If you are sure you have no thin objects (tree trunks are particularly bad, they'll snap right off), you could go down to 1 mm. That's the scale of the model shown here: http://www.flickr.com/photos/postapocalyptic/6606221371/in/pool-1862849@N21/