Beware of R16 your robot design may be too big.

[StevenB]

If your maximum horizontal dimension is 80", you bet you fit into an 80" cylinder. Maximum horizontal dimension doesn't mean straight down the side of your robot, it means the maximum dimension of your robot parallel to the floor. That is, it's the points with the greatest distance between them parallel to the floor on your robot.
So, in your example, it would be the bisecting line of your equilateral triangle.

I suspect that's what the GDC was thinking, but it's wrong. Note that the bisecting line of the equilateral triangle is 40" * sqrt(3) = 69".