John Gill Commentary 1 Kings 18:32

"And with the stones he built an altar in the name of Jehovah; and he made a trench about the altar, as great as would contain two measures of seed." — 1 Kings 18:32 (ASV)

And with the stones he built an altar in the same of the Lord ,
&c.] Whom the twelve tribes had formerly worshipped; and though now divided in their civil state, yet ought to be united in the worship of God:

and he made a trench about the altar, as great as would contain two
measures of seed ;
or two seahs, one of which was the third part of an ephah, and two of them were more than half a bushel; and this trench or ditch round the altar was as broad as such a measure of seed would sow.