instead of preparing and measuring an area, you could use a selection tool. as in Rate Calculator
https://mods.factorio.com/mod/RateCalculator

I have considered that. The thing is the size of MPF have to be pre-determined in the data.lua step, and can not adjust arbitrarily in-game. If we want to keep the shape of the zone unchanged then we need to restrict the zone in that shape.

you could use the next availabel size or show an error, that the area ist too big :-)
BTW I prefer sizes 30, 46, 62, that is what i can put in a Factory and 30, 62 is "chunkified" with a border of 1

you can contact me per MSG
here is no chat available ;-)