The problem isn’t with hardware or firmware limitations. It is mechanically quite easy to make a 40x40xm print surface. Software has a few values to define the sizes, so that is also quite easy. Technically it is possible, but the issue is that Plan B is very slow. It prints at 60mm/s with 2 sweeps per 3,2mm. Doing the math, 40x40cm will take around 28 minutes to print per layer. A layer is 0,15mm. This speed can’t be increased right now, this for now is it.
Because of that, I do not advice to make a printer that big based on Plan B. Once better hardware comes available for open source 3DP printing, it will become a more interesting idea to make a bigger printer.
Hope this helps.