In studying I Samuel 17:38-40, I am wondering if David is not some young “boy” as some stories depict him, but instead, is a young “man”.
Saul was the tallest man among the children of Israel and we see that he is now giving his armor to David to wear. David must have been around the same height and size of King Saul for him to give him his armor to wear. Of course, David wasn’t used to wearing armor and it would be a great disadvantage to him to try and fight in it. So, he goes out to meet Goliath armed with a sling and five stones.
David's brothers are in the battle against the Philistines. A man has to be at least 20 to be in the army (Numbers 1:3; 26:2). Since David is not with his brothers and it wasn't due to a lack of zeal, we must conclude that David is under the age of 20. David is the eighth son of Jesse. His three eldest brothers were in the army (I Samuel 17:14). David would be at least 5 years younger than the third son who is in the army. If we assume that the cut off between the third and fourth son was due to age, that would put David at 15 or less -- this is why he is depicted as a youth and so described: "And Saul said to David, "You are not able to go against this Philistine to fight with him; for you are a youth, and he a man of war from his youth."" (I Samuel 17:33). The Hebrew word for youth is na'ar which refers to someone between infancy and adolescence.
Nothing is said about David's height, but we do know that he was shorter than Saul because when Samuel was sent to anoint David, he assumed God would pick Eliab, David's eldest brother: "Do not look at his appearance or at the height of his stature, because I have refused him. For the LORD does not see as man sees; for man looks at the outward appearance, but the LORD looks at the heart" (I Samuel 16:7). Yet, David was a handsome youth. "Now he was ruddy, with bright eyes, and good-looking" (I Samuel 16:12). How tall David grew up to be, we are not told.
The armor doesn't necessarily tell us David's height. Not everyone in those days were clad from head to toe like the knights in England. Something as described in Ephesians 6 would be closer. And such armor would not depend on a man's height too greatly. However, a youth unaccustomed to carrying a shield and sword or spear would find them too heavy to use effectively.