Just thought of this while getting ready for bed.
Not sure if your coffee table would require tabs or such to keep the glass from shifting (one on each side, or an angle thingy on each corner) or not. The desk I saw had more flat surfaces, so the glass did not shift easily at all. I would get a piece of glass that will fit the whole top, and see if it will shift or stay put, before making any changes.
I would also get with a nearby glass shop, and see what their glass sizes are. Talk nice, and they should definitely work with you. Even the small town I live close to has a glass shop, and when I went in to see if I could get some scrap plate glass for Scary Sharp, they not only gave me some, but cut it, and didn't charge a cent. Nice people. And I was talking with the owner, not one of his crew.
Anyway, ask about sizes (they should be able to cut a large pane to sizes you want). You may have to make minor size changes to lessen waste, or may get lucky and they can cut a sheet up to sizes you can use with no waste at all.