There's 2 main types of 3D printer; the type that squirts out molten plastic from a nozzle, and one that uses thousands of layers of powder and a binder or laser to melt it together one layer at a time. The second one is much more versatile since you can get many kinds of plastic and some metals but these machines are far more complicated and expensive.
But the plastic nozzle type could still be useful for making patterns for casting, and if you're worried about surface finish or tolerances all you need to do is a bit of polishing after the print.
There's not really a limit to the size of printer you could make, if you can make a CNC controlled X-Y-Z table big enough you could print a whole car. When I looked into it the main issue with making your own is the stepper motors and their controllers. Printer parts are cheap but I think they lack the accuracy and torque you'd be after. Stepper motors can be had off ebay but the controllers are quite complex I think. I know a guy whose made his own CNC laser cutter which can also be converted to router/milling, table size about 400x600mm. He's currently working on a 1.4x1.4m version that he can convert to plasma cutting as well.
I didn't know Phizinza had made his own, definitely they guy to talk to on here for tips and tricks
. Seen one on the web a few weeks back, did laser cured resin type substance. much, much, much better, but at a price (around 3k from memory, very cheap for what it does though). 
