Adam Clarke on Exodus 27:1

Thou shalt make an altar - מזבח mizbeach, from זבח zabach, to slay: Septuagint, θυσιαστηριον, from θυσιαζω, to sacrifice or from θυω to kill, etc. See Clarke's note on Gen 8:20.

Four square - As this altar was five cubits long and five broad, and the cubit is reckoned to be twenty-one inches, hence it must have been eight feet nine inches square, and about five feet three inches in height, the amount of three cubits, taken at the same ratio.