John Gill Commentary Exodus 26:18

"And thou shalt make the boards for the tabernacle, twenty boards for the south side southward." — Exodus 26:18 (ASV)

And you shall make the boards for the tabernacle
As in the manner before described, so in number as follows:

twenty boards on the south side southward ;
which being a cubit and a half broad, made the length of the tabernacle fifteen yards according to the common account; but if these were cubits of twenty one inches, then its length was much greater.