Ammianus Marcellinus (16.10.15) writes of the impact of the Forum of Trajan on the emperor Constantius who visited Rome in 357 C.E., and conveys something of its grandeur:

But when he came to the Forum of Trajan, a structure which, in my opinion, is unique under the heavens, and a marvel which even wins the acceptance of the divine powers, he stopped in his tracks, astonished, while his mind tried to grasp the gigantic complex, which cannot be described by words and could never again be attempted by mortal men. He abandoned all hope of constructing anything of this sort but said that he only wanted to copy, as he was able to do so, Trajan's horse which was situated in the middle of the open court of the forum, and which carried the emperor himself.

Source: Ammianus Marcellinus, translated by John Rolfe...