twelve cubits

Exodus 27:1
You are to build an altar of acacia wood. The altar must be square, five cubits long, five cubits wide, and three cubits high.

2 Chronicles 4:1
He made a bronze altar twenty cubits long, twenty cubits wide, and ten cubits high.

Ezra 3:3
They set up the altar on its foundation and sacrificed burnt offerings upon it to the LORD--both the morning and evening burnt offerings--even though they feared the people of the land.

square

Exodus 38:1, 2
Bezalel constructed the altar of burnt offering from acacia wood. It was square, five cubits long, five cubits wide, and three cubits high. . . .