I need to know a bit more since IMO basements need windows and honestly glass block is not much safer than a good window with security glass and alot better light.

The size of the block will depend on the Size of the window, and if you use a vent. Typically they are made from more than one size so they fit the opening more precisely. I would recommend a prefabbed unit and install it as 1 vs trying to install it block by block. If you really want try it yourself dimensions would be needed to give you dimensions.

You can get a glass block window that is operable this way you can open it to air out or let fresh air in.