Deuteronomy 10:1 At that time the Lord said to me, “Chisel out two stone tablets like the first ones and come up to me on the mountain. Also make a wooden ark. a 2I will write on the tablets the words that were on the first tablets, which you broke. Then you are to put them in the ark.”
They wouldn't be too big for Moses to carry them up the mountain and smaller than the dimensions of the ark.
Bezalel made the ark of acacia wood--two and a half cubits long, a cubit and a half wide, and a cubit and a half high.
It was approximately 131×79×79 cm or 52×31×31 in.
According to the Talmud, the length and width of each of the Tablets was six Tefachim, and each was three Tefachim thick – roughly 50 and 25 centimetres (20 and 10 in) respectively
According to traditional teachings of Judaism in the Talmud, they were made of blue sapphire stone as a symbolic reminder of the sky, the heavens, and ultimately of God's throne. Many Torah scholars, however, have opined that the biblical sapir was, in fact, lapis lazuli (see Exodus 24:10, lapis lazuli is a possible alternate rendering of "sapphire" the stone pavement under God's feet when the intention to craft the tablets of the covenant is disclosed Exodus 24:12).