Based on a Gezeirah-Shavah "Ravu'a" "Ravu'a" from the Mizbe'ach ha'Ketores. And the Torah gives the height as three Amos, because that was the height of the Mizbe'ach (excluding the K'ranos) above the Sovev - a walkway for Kohanim around the Mizbe'ach (Rashi). Refer also to Bamidbar, 3:27:1:1.
3)
Having given the measurements as five Amos by five Amos, why does the Torah add the word "Ravu'a"?
Oznayim la'Torah: Square is not dependent only on the four sides being equal, but also on the four corners, which must all be at an angle of ninety degrees.
4)
What if the Mizbe'ach was not exactly square?
1.
Zevachim, 62a: Wherever the Torah inserts the word "ha'Mizbe'ach", it is crucial to the Mitzvah,
1
and without it, the Mizbe'ach is Pasul.
2
The same D'rashah pertains in Tetzaveh, 29:13 to the K'ranos and to the Y'sod there in Pasuk 12, and in Tzav, Vayikra 6:7, to the Kevesh. See Torah Temimah, note 5.
QUESTIONS ON RASHI
5)
Rashi writes that one opinion holds that the Mizbe'ach was ten Amos tall. How could the Kohanim place blood on the corners, and wood, limbs and Cheilev on the fire on top?
1.
Rashi (at the end of Yisro) and Da'as Zekenim: They ascended to the top of the Mizbe'ach via a ramp.
1
Even though the Torah does not mention a ramp, it must have been a ramp - and not steps - since one may not ascend the Mizbe'ach through steps. Refer to 20:23:1:1.
