For the pitch grid I find each individual box to be 2 ft per side and the entire grid is 10yds per side and approx 14 yds on the diagonal.
The approach grid is a different story the boxes are 3 yards wide, I know they don't look it, 9 boxes a side making 27 yards across and 38 approx on the diagonal.
The problem with the full shot approach grid is your marker does stay in the centre when you move it depending on the topography of the green. It's ok if the green is flat, it stays centred but on a slopey green you need to be careful if your using it to measure. The grid it's self is correct but your marker will be pulled off from the centre.
This is what I believe is happening, I'd be grateful for some confirmation though..lol.
Edit: The approach grid does not remain accurate it wraps itself around the topography and gets distorted. I think the pitch grid does the same but it's less noticeable.