Exodus 27:1

And thou shall make an altar of shittim wood
This is a different altar from that made of earth before the tabernacle was built, ( Exodus 20:24 ) and from the altar of incense, ( Exodus 30:1 ) this was to offer burnt offerings on, and was placed at the door of the tabernacle, in the court of the people, where they brought their sacrifices to the priests to offer for them: it stood in the open air, as it was proper it should, that the smoke or the sacrifices might ascend up and scatter. This altar was not typical of the altar of the heart; though indeed all the saints are priests, and every sacrifice of theirs should come from the heart, and particularly love, which is more than all burnt offerings; but the heart is not this altar of brass to bear the fire of divine wrath, which none can endure; nor does it sanctify the gift, it being itself impure: nor of the Lord's table, or the table on which the Lord's supper is set; that is a table, and not an altar, a feast, and not a sacrifice; is not greater than the gift, nor does it sanctify: nor of the cross or Christ, on which he died, bore the sins or his people, and sanctified them by his blood; but of Christ himself, who by his office as a priest, his human nature is the sacrifice, and his divine nature the altar; and he is that altar believers in him have a right to eat of, ( Hebrews 13:10 ) his divine nature is greater than the human, is the support of it, which sanctifies and gives it virtue as a sacrifice, and which makes the sacrifices of all his people acceptable to God. This altar of burnt offering is said to be made of "shittim wood", a wood incorruptible and durable; Christ, as God, is from everlasting to everlasting; as man, though he once died, he now lives for evermore, and never did or will see corruption; his priesthood is an unchangeable priesthood, and passes not from one to another, and particularly his sacrifice is of a continual virtue and efficacy:

five cubits long, and five cubits broad:
the altar shall be square: as to the length and breadth of it, which were alike, two yards and a half each, according to the common notion of a cubit. The altars of the Heathens were made in imitation of this, they were square as this was. Pausanias makes mention of an altar of Diana, that was (tetragwnov) "square", sensibly rising up on high. And this figure may denote the perfection of Christ's sacrifice, and the permanency of it; though the altars in Solomon's temple, and in the visions of Ezekiel, are much larger, and which also were square, ( 2 Chronicles 4:1 ) ( Ezekiel 43:16 ) . Christ's sacrifice is large and extensive, making satisfaction for all his people, and for all their sins; and he is an altar large enough for all their sacrifices to be offered up to God with acceptance:

and the height thereof shall be three cubits;
a proper height for a man to minister at; for as Aben Ezra observes, the height of a man is but four cubits ordinarily; so that a man serving at the altar would be a cubit, or half a yard more above it, and would have command of doing on it what he had to do.