The average adult height for men is 5'7" - 5'9" and 5'2" - 5'4" for women.  However, height varies significantly by country and region. For example, men in Northern Europe, such as the Netherlands and Scandinavia, often exceed 180 cm (5’11″), while men in parts of Southeast Asia average closer to 162 cm (5’4″). Women follow a similar pattern, with Northern European women averaging around 168 cm (5'6") and South Asian women closer to 152 cm (5').

Genetics factors play a good part in determining a person's height. So with Adam & Eve falling within the normal average height for men & women where did the giants get their height from?  The Bible tells us that the sons of Elohim (God) went into the daughters of women, and bare a race of giants.  The giants must have gotten their height from their fathers the fallen angels, who are called the sons of Elohim (God) who created them. 

According to 1 Kings 6:23-28, the two cherubim angels inside the most holy place inside of the tabernacle and later on in the temple who covered the ark of the covenant were both 10 cubits in height which is 15' in height, and one wing was 5 cubits (7 1/2') and the other wing was 5 cubits (7 1/2'). Hebrews 9:23 tells us that the things in the tabernacle were patterns of things in the heavens. So these two 15 ft. cherubim angels were a pattern of the size of the cherubim angels in heaven.  They were gigantic in height and size which means the fallen angels who went into the daughters of men and procreated the giants, who were also gigantic in height and size. The giants didn't get their height and size from their mothers who were the daughters of men; they got their height and size from their fathers, the fallen angels. 

This article Heavenly protection in history: Two remarkable true stories of angelic intervention – USSA News | The Tea Party's Front Page. describes the angels in the first story as hundreds of big men with shining garments and drawn swords, and the second story describes an angel as a towering angelic figure with golden hair, wielding a great sword.

Please consider the fact that angels can also shapeshift their gigantic figures down to the size of a human man as shown to us in Genesis 19:1-5 when two angels visited Lot in Sodom and the men of Sodom thought the two angels were men. Also the spirits of the giants on Earth called the demons can likewise shapeshift what they looked like as a giant to appear as a familiar spirit of a person whom we know which means the giants got their ability to shapeshift their appearance from their fathers, the fallen angels because they certainly didn't get this ability from their mothers, the daughters of men or the daughters of Cain.