Well, the 2020 extrusion as with any material will be less rigid the longer that it is. Longer material has a greater moment arm when any force is applied to it, so it becomes easier to bend, and relatively weaker as well, so the amount of flex that it will have with any applied force grows exponentially. This is a reason to make the printer as small as possible while still able to manage what you do with it.
You can increase the rigidity of the frame by adding side panels which anchor to the frame and help support it, but this is of course at the cost of additional material and mass. Speaking of mass, the longer the X rail is the more it weighs and flexes as well. The MP3DPv4 uses an unsupported MGN12H rail for the carriage, so at some point this will become a limiting factor. Many CoreXY printers support the linear rail with an additional extrusion, but this also affects mass and limits the possible acceleration of the print head, mostly in the Y dimension.
IMHO, 300mm isn’t unfeasable. I have a larger volume on my MP3DPv3 and get excellent results, even though the gantry is even more flexible than this one. But it is a reason to keep the build volume as small as practical.
To be useful, I think that a 200mm cube build volume is a good target for all-purpose 3D printing. You can already slice a print that will take days in that volume. I have very seldom needed a larger build volume. The reason my Repeat build volume is larger is because of the parts I had ready mostly (A CR-10 build plate which is 300mm by 300mm usable area) as well as the MGN12H linear rails that I could easily obtain, and that I am unwilling to cut. Same with my MP3DP v4, where I had 300mm rails for Y and Z and got a 350mm rail for X to make for a 250mm cube build volume – this time I have a 235mm square build plate. I should probably have cut the rails down and just used 200mm anyway, particularly since I have the Repeat if I need the extra size. I am certain that the 235mm build plate would have fit inside the machine. Well, I can’t resist changing stuff, that’s why I DIY.