How come Saul’s armor fitted David?
In 1 Samuel 7:38-40, Saul gives David his armor to fight Goliath because he cannot sway the young man otherwise. The Bible tells us that Saul was head and shoulders above everyone around him in Israel when he became king (1 Samuel 10:23).
To my knowledge, the Bible never tells us how tall David was. It does describe him as ruddy and handsome (1 Samuel 16:12). David doesn’t take off Saul’s armor because it doesn’t fit him. At least, the text does not say that.
David’s reason for not wearing Saul’s armor into battle with Goliath is that he had not tested it on his own. He was in a hurry to beat Goliath. And he wanted to do it his way. Instead of armor, he came against Goliath as a shepherd.
The Bible tells us that he came against Goliath and battle with only a shepherds staff, five stones, and a sling. Of course, God anointed him so that the Holy Spirit was also with him in battle. The Holy Spirit was the one who gave him the ability to kill Goliath.