There are several accounts of giants in the bible. The Book of Job 16:14 says that He breaketh me with breach upon breach, he runneth upon me like a giant, in which case the term is clearly being used as a metaphor. But in plenty of other passages throughout the Old Testament (the term never surfaces in the New), it is used as if we were speaking about real creatures.
Even if we put our initial mythological imaginings aside, the authors of the King James bible indeed seem to be referring to are very large people. But how large? There were giants in the earth in those days, says Genesis 6:4. And there we saw the giants, the sons of Anak, which come of the giants: and we were in our own sight as grasshoppers, and so we were in their sight, says Numbers 13:33.
But we don’t necessarily have to stick to the text of the bible for more of these accounts, and even some anthropological data on them. For one, much secular scientific data suggests that human beings – in general – get smaller the father you go back in time. Our diets, lifestyles and DNA all seem to have put us on the track of growing larger over the years. So if we assume this to be true, then perhaps these giants, such as the Zamzummims of Deuteronomy 2:20 or those born unto the giants in 1 Chronicles, wouldn’t be so breathtakingly large by today’s standards.
Goliath, perhaps the most infamous of the “giants” first appearing in 1 Samuel 17:4, is referred to as six cubits and a span, which most modern scholars agree would be about 9 foot 9 inches tall. Of course, the six cubits here could be a literary exaggeration, as all earlier texts, including the dead sea scrolls, all list his height at 4 cubits and a span, or 6 foot 9 inches tall.
In any case, much like the unicorn also mentioned in the bible, or dragons, the idea of giant in these texts isn’t necessarily as fantastic as it may seem at first. The same goes for the mention of wizards in the bible, which is used in the context of those who practice witchcraft and black magic ceremonies, not the wizards with purple pointy hats we tend to think of today.