What is the size of the hole down the center of the ground tube?

The diameter of the hole in the stainless steel tube, in which the solid stainless steel rod of the guide slides into, is 0.55 inches, or 14mm.

What is the outside diameter of the ground stake/tube?

The outside of the tube is 18mm (+/-0.3mm), so it is 0.7 inches to 0.72 inches.  For installation into concrete, use a 3/4-inch masonry bit to drill a hole at least 13 inches deep as the tube is 12-3/4 inches long.  Then, we suggest adding some silicone caulk to the outside of the tube, but make sure the hole in the bottom tip of the tube stays clear so water can drain. The silicone caulk will help keep the tube from lifting out.

What grade of stainless steel is this item made of?

It is AISI 304 stainless steel.