Torah mentions a nation of very tall people and provides some information about their size. Let's take a look:
YLT Dt 3:11 for only Og king of Bashan had been left of the remnant of the Rephaim; lo, his bedstead is a bedstead of iron; is it not in Rabbath of the sons of Ammon? nine cubits its length, and four cubits its breadth, by the cubit of a man.
First of all, who are Rephaim? Simple cross-reference points out to Genesis 14:5, Deuteronomy 1:28 and Deuteronomy 2:10-11
YLT Gn 14:5 And in the fourteenth year came Chedorlaomer, and the kings who are with him, and they smite the Rephaim in Ashteroth Karnaim, and the Zuzim in Ham, and the Emim in Shaveh Kiriathaim,
YLT Dt 1:28 whither are we going up? our brethren have melted our heart, saying, A people greater and taller than we, cities great and fenced to heaven, and also sons of Anakim--we have seen there.
YLT Dt 2:10 'The Emim formerly have dwelt in it, a people great, and numerous, and tall, as the Anakim;
YLT Dt 2:11 Rephaim they are reckoned, they also, as the Anakim; and the Moabites call them Emim.
As you can see, it was some ancient nation that was tall, great and numerous. Sons of this nation sacred the spies that Moses send out to spy out the land of Canaan.
Secondly, 9 cubits is 13.5 feet based on 18 inch cubit. According to Wikipedia, the tallest verified man in the world was Robert Wadlow, who was 8 feet 11 inches at his death. This is almost 6 cubits. Normal people, as shown in the picture below are 4 cubits on average.
As you can see, if we, humans, can reach 6 cubit height, the sons of ancient nations could've been 9 cubits tall. And judging by a picture above, no wonder that Hebrews were scared of such tall and great people.