Seeing this Eustathios Spider v2 in action- I think it would be cool to make a larger 40x40cm version - and somewhat simple: you would just need to add 10cm to the horizontal extrusions and linear rails. You could even add 10cm more to the Z. - In fact a person could probably add a lot more to the Z if they wanted to- without any performance issues- I mostly don’t need a large Z for the work I do- but it seems possible.
As things scale up, you have greater chance of deflection, so components have to be made more rigid. Which adds weight. For example on the HercuLien it has a usable 338 X 358 X 320. I went with 10mm cross rods. That could be increased with a more compact carriage (since the 50mm blower fans hang off the back). But 400mm is getting pretty huge, ask anyone who has a HercuLien. For that I would look into using ground linear guides on the sides instead of rods. You could still couple the sides to drive off one stepper with a common drive shaft at each end. Look into what @Tim_Rastall was working on before he went into hiding… aka back to the real world 
when you say deflection, are you saying the force of gravity pulls down the middle of the cross rods so the printing level would vary/bow near the middle? I can see that the M8 cross rods can bend relatively easily, hopefully M10 cross rods would be enough to add 10cm more in X and Y. Interesting that linear guides on the perimeter would provide more stability than the M10 rods- then the belt tensioner with the belt attaches to the guide. It’s hard to believe that 10cm would make a huge difference- it’s just going to be a thought experiment for a long time since I’m moving onto to other projects. I will look at Tim Rastall’s work- thanks for the tip!
Deflection due to forces on the rods increases with the cube of length, so yeah, fairly modest length increases can add a LOT of flex.
@Ted_Huntington what also becomes an issue is rods on the edges remaining straight over longer distances. Since these are both a bearing surface (translation) and a drive train for the opposing axis (rotation) longer distances can begin to create harmonics that can cause issues.
I guess a good person to ask is @Ashley_Webster , I think he had plans to construct the largest printer of this style so far… But I am not sure how it is running.
ok thanks for the explanation and pointer to Ashley Webster- I will take a look.
@Ashley_Webster
oh ok I see- is that at Herculean or Eustathios you are working on? With the standard Eustathios Spider V2 I am not seeing any deflection- but I just started printing and making small objects- it seems to me that 10cm more would not drastically change matters- I would probably change the M8 carriage rods to M10 just to be on the safe side- but I wonder if I really would have to.
oh ok thanks