I am in the final stages of building a large printer. Don't ask me why.... I think the biggest challenge will be flatness of the bed. It needs to be within a few thousandths of flat. Mine will print a 21" cube. I milled the bed flat, but I am not sure it will be good enough. About all I have left is wiring it so I should find out soon.
I am using Nema 34 steppers on the y and z axis and a Nema 23 on the x. I am using a ramps 1.4 controller. Instead of using the traditional stepper driver modules, I hacked the step and direction signals and feed them to standalone stepper driver modules. Speed may be an issue. I put together a test circuit wit an Arduino to move each axis. I didn't do any analysis on motor size. I just used some motors, drivers, and precision ball screws I already had. I could reliably move the axis at 100mm/sec. I'm not sure if I can tune it up much faster than that though.
I did a couple of novel things relative to mounting the extruders. It has dual extruders. Each one is mounted on its own bearing block on a miniature linear slide. In other words, the extruders are on independent carriages. One extruder is attached to the ball nut. The other extruder "sticks" to the first with magnets. That would be for traditional dual extruder mode. The second extruder can be pulled apart from the first and a 10" beam connected between them. In this mode, you could print the same file out both extruders at the same time and print two parts at once.
I'm not sure where I'm going to get a 24x24 piece of flat borosilicate glass.