3/4'' is the size of the stamped out diamond on the shortest section and it's 9 gauge steel. There's been a few guys that converted the GF's to metric so they can tell you what they used.
I really don't think it is critical that the size of the opening in the expanded metal matches the plans exactly. Get whatever the local steel supplier has that is closest. It is after only to hold the meat being cooked/smoked. Just remember this is a bbq smoker not a NASA space probe.
Just make sure it is not too flimsy. As RC and FC will attest, it can never be too thick...
Learning generally boils down to "Repetition or the avoidance of pain", some people learn by doing, some by watching and some just have to pee on the electric fence.
Whether you think that you can, or that you can't, you are usually right. Henry Ford