One of the biggest issues with 3D printing, outside of printer reliability, software stability, and finding the right combination of speed and extrusion rate is build volume.  I have a Makerbot 5th Generation and as LARGE as the machine appears to be, the total build volume is probably about 1/5 the machine's total INTERNAL bulkiness.  DIY printers are usually not much larger, unless you are a SERIOUS builder.