A 1 oz Gold coin would normally be smaller than a 1 oz Silver. I'm guessing for a 1/2 oz Gold, this would be around 27mm diameter or less. So it would be smaller.
My guess would also be 27 mm, 39.2 mm like the other coins is out of the question.
27.0 mm will result in a thickness of 1.85 mm, for a 1/2 Oz coin.
I have a couple of 1/2 cold coins (world coins), you can really "feel" the density of gold when holding a 1/2 Oz coin in your hand.
I am still fascinated of the density of gold, its heavy!
Edit: The hologram is ~25 mm