I can't remember if I used 2 or 3 packets of Knox brand gelatin, but it made a 9x13 pan about 1" deep.
I followed the directions on the box, but substituted white grape juice and about 2 Tbsp. sugar instead of water, just to give them some flavor.
turn out slightly yellow, like the juice, but once I put them on the cake, it didn't make a difference, and I think they look a little more realistic since they can be made square like a real ice cube.
Good luck! Let me know if you have any other questions.