Charles Ellicott Commentary 2 Samuel 23:21

"And he slew an Egyptian, a goodly man: and the Egyptian had a spear in his hand; but he went down to him with a staff, and plucked the spear out of the Egyptian`s hand, and slew him with his own spear." — 2 Samuel 23:21 (ASV)

A goodly man. —The meaning is explained in the parallel place in Chronicles, where he is called a man of stature, and it is added five cubits high. Benaiah’s exploit, therefore, consisted in coming, armed only with a staff, to this giant Egyptian, wresting his spear from him, and then slaying him with it.