I am in the final stages of completing my MPCNC build. I opted for a frame size of 1200mm * 800mm (so tube lengths of 1200mm and 800mm). I comfortably fitted a 900mm * 600mm (this was an off the shelf 16mm MDF board size at my local hardware store) spill board, and when I decked the spill board I settled on a usable bed size of 800mm * 400mm. I could have pushed this out further to a maximum of approximately 850mm * 450mm (maybe 480mm) before I started running to the end of the gt2 belts.
Is this any help?
At these lengths I don’t think I haven’t encountered any rigidity issues (early days yet) but as you say its mainly foam, I think you could problably go even bigger again if you need to.