3,842. The Indispensable Aspects of the Altar

Hilchos Beis HaBechirah 2:17

The altar absolutely must have four horns, a base and a square shape; if any of these or the ramp is missing, the altar is unfit for use. However, the dimensions of length, width and height, are not indispensable so long as they are at least one cubit long, one cubit wide and three cubits tall. These were the dimensions of the woodpile on the altar in the wilderness.

Hilchos Beis HaBechirah 2:18

If a handbreadth of the altar is damaged, it is rendered unfit; if less than a handbreadth is damaged, it remains fit so long as none of the stones are damaged.