Joint thickness for glass block spacers
This site says 4” modules in both directions. (I know some glass blocks come in 6” modules.) 

https://m.lowes.com/pd/SEVES-24-Pack-2-375-in-x-4-in-Non-directional-Glass-Block-Spacer/1000117677

So to answer your question, the mortar joint is going to be as wide as necessary to provide a 4” module. (The spacer will make the joint the correct width.)

Don’t forget to add one mortar joint width for your wood frame