If Shaq, a well built man proportionally speaking, was 300 / 320 pounds at 7'1" feet tall, you can bet Goliath, carrying around on him a 150 pound metal armor, and a throwing a spear with 25 pound spear head (try throwing a 25 pound olympic sized plate any distance for comparison) was around 9 feet and 6 inches or so tall. You can also believe that he was easily tipping the scales at 450 pounds or maybe even more. Remember that in the worlds strongest men contests you have men that are 6 feet and 2 or 3 or 4 inches tall that weigh around 300 to 375 pounds and even more.
See bio for Brian Shaw. He is 6'8' and weighs 420 pounds.
https://en.wikipedia.org/wiki/Brian_Shaw_(strongman)
So it is no stretch to imagine that Goliath at 9 feet 6 inches tall could have weighed as much as 600 to 700 pounds. With a armor of 150 pounds and a spear head that weighed 25 pounds, I actually think he was closer to the 600 to 700 pound weight then the 450 pound weight, although naturally you can not say for sure. But you can make a educated guess.
To put it another way, this guy was not to much smaller then a healthy bear, and maybe even bigger then some of adult bears. Maybe as strong as some of them to.
You don't think all of Israel wouldn't be scared of this monster?
Again, to look at it another way, the NBA, full of strong and tall and physically gifted men, would not take on Shaq or Wilt in a fight. None of them would. Yet in this case your talking about guys that are usually "only" 5 to 8 inches smaller then a Shaq or Wilt. But all the NBA knew how devestatingly strong they were.
So now imagine a average sized Israeli at around 5'10" and 200 pounds. How, without the courage of God in them, could they not be frightened so much that they probably wished that this nightmare of a man would just go away.
This is why David was such a hero.