It seems like an easy enough thing to just have an Arduino some thermistors and some SSRs that would control as many heaters as you want. Getting it controlled via the Marlin board is the harder part. Making something that always heated to 205C would be about 15 lines of arduino code.
Do it like the laser, remap to a 5V pin, read the pwm value with the other arduino have in controll all heater independently. Wire up the filament sensor in marlin, do some sort of mechanically triggered spool feeder or a bowden with a bigger stepper.
That reminds me I have a new extruder to put up. High temp MK8???
The largest heat bed I’ve seen is a 500mm x 500m, silicon pad with nearly a kW of power required, using 220V AC. I expect there could be another running on 110V though. There are plenty of standalone PID controllers to run beds that large though I’m not sure how best to control an external PID with the RAMPS. You’d want to use a separate circuit for the heat from the circuit for the rest of the hardware, and if you were going to do 110V you’d need a separate circuit for each heat zone…
By far it’s easiest to build in multiple pieces and assemble using glue.
what’s the largest dimensions you guy’s have built an MPCNC and had satisfactory results with 3d printing?
Mike that is a pretty subjective question, I think Dui has done some of the largest prints and he seemed happy with them.