ANSWER: First, we need to cover how BIG and STRONG was Goliath the Giant that challenged Israel and battled David. He was, of course, a giant, from the city of Gath. The Bible states he was six cubits and a span tall (1Samuel 17:4).
The exact length of the ancient cubit, using modern measurement standards, varies depending on the source used. Various Bible commentaries place the length of a cubit anywhere from 43 centimeters to 53 centimeters. The length of a span is believed to have been around 23 centimeters. Goliath, the giant of Gath, was easily 2.8 meters tall and likely bigger (see 1Samuel 17:5 - 7).
Goliath wore a protective coat that weighed at least 78 U.S. pounds (35.4 kilograms). Several commentaries place the weight of the coat at an amazing 156 U.S. pounds (70.8 kilograms)! The length of his spear may have been as long as 26 feet (7.9 meters). The head alone on his spear weighed at least 17 pounds U.S. (7.7 kilograms). He was a formidable opponent indeed!
It's interesting that you mentioned the coincidence of picking up five slingshot stones and Goliath having four similarly gigantic brothers. Now we know from 2Samuel 21:16 - 22, especially the last verse, that there were at least four other Philistine giants. There is, however, no record of these other four challenging the army of Israel.
What's interesting, from the viewpoint of seeing this encounter as a lesson in faith, is that David took five stones (1Samuel 17:40), not just one, despite having faith in God and His special protection. He prudently planned that it might take more than one stone to kill the giant man, much as Jacob carefully prepared before encountering his brother Esau again after leaving the employ of Laban (Genesis 32 - 33) while praying to God before this meeting. He also wisely choose smooth stones in order to maximize their accuracy and minimize their resistance to the air. Other shapes of stones would have traveled through the air slower and less accurately.
There is an interesting translation issue in 2Samuel 21:15 - 21 in that the term translated "the giant," in the plural, can mean "Rephaim." This apparently was a race or breed of giants, such as Og, the king of Bashan, who had a bedstead of about 13 1/2 feet in length and 6 feet wide (Deuteronomy 3:3, 11; cf. Joshua 12:4). Although one of these four other Philistine giants WAS Goliath's brother (2Samuel 21:19; cf. 1Chronicles 20:5, which preserved the text more correctly), the others aren't clearly said to be.
Another giant race in the Bible are called the Anakim (Numbers 13:28, 32 - 33), but it doesn't appear they were related to the Philistines whom David fought.