Wow thats pretty large. Glass and grout adds a lot of weight and might possibly compromise the windows strength. I would want to consult an engineer before I did something that large. Perhaps doing somewhat smaller pieces in frames (like say regular windows) and mounting them on chains to hang in front of the windows is an option.
On second thought-are the windows purely that size altogether -one large piece of glass? Or broken up-several windows adding up to that size?

It is one large window. Yes I thought of doing smaller ones to hang in front of the window. What is the largest you have worked with before?

No where near that size LOL-regular window size. But even then you can see how the weight affects it. Stained glass panels often need reinforcement over a certain size. You are probably better off with hanging pieces-and hey, they can be moved in the future if needed. Its horrible to see a mosaic destroyed to change a buidling.

Consider Plexiglass panels. You would use the thicker size, I think it is 1/4", and I believe they come in 4'x8' panels. Lowes carries it and they will cut sizes to fit your specifications, as long as it's just a straight cut. You can drill holes in Plexiglass to install a hanging mechanism. It weighs less than glass and won't be accidentally broken.