the shaft zincs are 1.25", I use two on the shaft...the strut zinc is at least 2.75" (or larger) but I did not have the diameter handy. Perhaps someone else can get a number for you.
There has been a lot of junk zinc for sale, so make sure the zincs are actually zinc or they won't last a month and you will be replacing them.
FYI there is a wide spot on the strut where a fairly large diameter flat, round zinc is attached.