Does anyone know if there is a reason for using 3 different tube sizes for the roll bar?
Couldn't it all be done in one size?

I've only got two, roll bar in around 51mm and cross bar and stays about 40mm.

I think one size would look to chunky on the supports.
But it s up to you as it's your build

I've done the same as Landy and used two sizes of tubes.

I suspect three sizes were used originally because that's what Chris happened to have lying around! It was built on a budget afterall :D

How about the whole thing in 42mm?

I was thinking it would be more equal. I can't find any pictures of anyone doing this but may give it a try