Therefore said we, that it shall be, when they should so say to us or to our generations in time to come, that we may say again, Behold the pattern of the altar of the Lord, which our fathers made, not for burnt offerings, nor for sacrifices; but it is a witness between us and you. (Joshua 22:28)
The altar was an exact representation of the altar at Shiloh and would be a witness that its builders acknowledged and served the same God as those who worshiped at the original altar. Probably the size was much larger, however, to render it conspicuous, but the construction and proportionate measurements were the same.