The touch plates are approximately 2 1/2 inches square overall.
The milled out area is approximately 2 inches square which leave 1/2 inch un-milled on the bottom side.
The total height is approximately 1 inch with the milled out area being just under .8 inches.
You can use it on material that is .20 inches thick or greater and yes you can flip it upside down and use it to zero the z axis anywhere on the material.
I have made custom sizes in the past on request for those milling out PCB's or other very thin material.
Hope that answers all your questions. If you have more feel free to ask,