He slew an Egyptian - This man in Ch1 11:23 is stated to have been five cubits high, about seven feet six inches.
He went down to him with a staff - I have known men who, with a staff only for their defense, could render the sword of the best practiced soldier of no use to him. I have seen even a parallel instance of a man with his staff being attacked by a soldier with his hanger; he soon beat the weapon out of the soldier's hand, and could easily have slain him with his own sword.
We have a good elucidation of this in a duel between Dioxippus the Athenian and Horratas a Macedonian, before Alexander: "The Macedonian, proud of his military skill, treated the naked Athenian with contempt, and then challenged him to fight with him the ensuing day. The Macedonian came armed cap-a-pie to the place; on his left arm he had a brazen shield, and in the same hand a spear called sarissa; he had a javelin in his right hand, and a sword girded on his side; in short, he appeared armed as though he were going to contend with a host. Dioxippus came into the field with a chaplet on his head, a purple sash on his left arm, his body naked, smeared over with oil, and in his right hand a strong knotty club, (dextra validum nodosumque stipitem praeferebat). Horratas, supposing he could easily kill his antagonist while at a distance, threw his javelin, which Dioxippus, suddenly stooping, dexterously avoided, and, before Horratas could transfer the spear from his left to his right hand, sprang forward, and with one blow of his club, broke it in two. The Macedonian being deprived of both his spears, began to draw his sword; but before he could draw it out Dioxippus seized him, tripped up his heels, and threw him with great violence on the ground, (pedibus repente subductis arietavit in terram). He then put his foot on his neck, drew out his sword, and lifting up his club, was about to dash out the brains of the overthrown champion, had he not been prevented by the king." - Q. Curt. lib. ix., cap. 7.
How similar are the two cases! He went down to him with a staff, and plucked the spear out of the Egyptian's hands, and slew him with his own spear. Benaiah appears to have been just such another clubsman as Dioxippus.

And he slew an Egyptian, a goodly man,.... A person of good countenance and shape, very large and tall; in Ch1 11:28, he is said to be a man of great stature, and five cubits high, and so wanted a cubit and a span of the height of Goliath, Sa1 17:4,

and the Egyptian had a spear in his hand; as large as Goliath's; for in Ch1 11:23, it is said to be like a weaver's beam, as Goliath's was; see Gill on Sa1 17:7,

and he went down to him with a staff; with a walking staff only, having no other weapon:

and plucked the spear out of the Egyptian's hand; and therefore must be a man very nimble and dexterous, as well as bold and courageous:

and slew him with his own spear; as David cut off Goliath's head with his own sword. This is supposed (l) to be Shimei, the son of Gera, Kg1 2:46.

(l) Hieron. Trad. Heb. ut supra. (in. 2 Reg. fol. 80. C.)