What limits the model size? Cost and time. Models are limited by how accurate the machine's physical rails are, etc. The longer and straighter they are, the more expensive. There are a FEW machines that can do larger format work, like say up to 2 or 3 feet per side but they start at a million bucks and go up from there. And their quality isn't that impressive either.

The larger the model, the longer it takes to print. Some really large models for me (meaning full sized - 13" x 13" x 8") might take nearly 3 days to produce.