RepRap stands for Replicating Rapid Prototyper and is also the name of the group aiming to create an open source machine that can do just that. Find out more at RepRap.org. This is our (Me, Scott, and Joel) experience creating that machine.
The very first thing we decided to do was create the frame. Wood is cheap and plentiful, and since we scavenged a bunch of loft wood from the dorms last we had tons. We needed to decide on how our design would be set up. After drawing up some pictures, and discussing it for a while, we decided to split the stages into two parts. The top of the machine would be the X stage which would hold the print head, and the bottom part would hold the YZ stage.
Our linear stages would be using smooth rod scavenged from a broken copy machine to hold the stages. We made the inner dimensions of our frame the same as our rods, although now I would suggest making them an inch or so smaller so you dont need to add extra boards to mount the rods on. As for height its not too important, but we used something like 3 feet.