- The luchot are a 1 amah cube of sapphire (6x6x6 tefachim) (Baba Basra 14a)
- 3x6x6 tefachim individually (Baba Basra 14a)
- The writing filled each side ("tradition". I think I saw this in a Gemara too)
- There are more words in the first 5 commandments, so the letters were a smaller size to fit.(Mabit)
- The letters were carved straight through the luchot. (Shmot 32:15)
- The commandments may have been carved on all 6 sides of the cube. (Chikur HaDin II, ch. 2)
They may have looked something like this:
My 3D animation according to Talmud Yerushalmi Click to enlarge: