The simplest and most accurate way to measure the diameter of a tube is to use a set of calipers if they are available.
In the absence of a set of calipers an adjustable wrench and tape measure or ruler can be used. As illustrated in the photo adjust the adjustable wrench until the jaws touch both sides of the tube.
Once the jaws of the wrench are touching both sides of the tube, measure the distance between the jaws of the wrench with a tape measure or ruler. The resulting measurement equals the diameter of the tube.
To get the center to center dimension of the pedestal guard, measure from the inside of one of the vertical tubes to the outside of the opposite vertical tube. As illustrated in the photo you can see that the center to center spacing of this pedestal guard in 9 1/2 inches.