פּירוש אויף שמות 27:1

ועשית את המזבח וגו׳ ושלש אמות קמתו AND THOU SHALT MAKE AN ALTAR etc. AND THE HEIGHT THEREOF SHALL BE THREE CUBITS — These words should be understood as they are written (i. e. to mean exactly what they say — that its height was three cubits); this is the opinion of R. Jehudah. R. José, however, says: it is stated here, “[the altar shall be] square” and it is stated with reference to the inner (the golden) altar, (Exodus 30:2) “square [shall it be].” How was it there (in the case of the latter)? Its height was twice its length (“a cubit the length thereof, and a cubit the breadth thereof, . . . and two cubits the height thereof”)! So, also, here: the height shall be twice its length, i. e. ten cubits. And if you ask, how, then, do I explain the words and three cubits the height thereof?” I reply that this gives the measurement from the edge of the surround (סובב) upwards (whilst 10 cubits was the actual height from the ground to the top of the altar) (cf. Zevachim 60a; Rashi on v. 5).

'ועשית את המזבח וגו, “you are to construct the altar, etc.;” the word מזבח here may be understood as an acrostic of the words: מחילה, זכות, ברכה, חיים, “forgiveness, merit, blessing, life.”

"And you shall make the altar of acacia wood." In the Midrash Tanchuma (10), it concludes: "They committed folly [שטות] and angered Me with the [golden] calf, so let the acacia [שטים] wood come and atone for their folly." This is because a spirit of folly [שטות] enters every sinner (Sotah 3a). For this reason, Solomon refers to the wicked as fools, and therefore it says regarding the altar, "hollow with boards shall you make it," because anyone who is without knowledge and understanding is called a hollow man [נבוב], as it is said (Job 11:12), Let the hollow man gain understanding - Rashi explains there that anyone who is empty and hollow without knowledge and understanding needs to acquire a heart to return in repentance.
And its height shall be three cubits - like the height of an average person. And five cubits in length and five cubits in width - making ten, corresponding to the ten things that come from a father and mother: flesh, blood, sinews, etc., as enumerated in Tractate Niddah (31a). And through these comes all sin and iniquity, and this is [also indicated by] the half-shekel of ten gerahs.
And you shall make its horns - to atone for the sinner who is compared to a ram with horns that gores upward, as it is written (Psalms 75:5-6), And to the wicked, "Do not lift up the horn; do not lift up your horn on high", as explained above in Parshat Vayera in the verse And behold, a ram caught afterwards.
And you shall overlay it with copper - Rashi explains, to atone for brazenness, as it is said [with reference to brazenness] (Isaiah 48:4), and your forehead is copper. This agrees with our explanation, since every horn is on the forehead.