639.84 cm is equal to 251.9685 inches.
To put this into perspective, here are a few comparisons to known objects of similar size:
1. An average adult male hand span is approximately 7 inches, so 251.9685 inches is equivalent to the length of about 36 hand spans.
2. The average height of a kitchen counter is around 36 inches, so 251.9685 inches is just under 7 kitchen counter heights.
3. The length of an average dollar bill is 6.14 inches, so 251.9685 inches is approximately equal to the length of 41 dollar bills placed end to end.
4. The wingspan of a bald eagle can range from 71-96 inches, so 251.9685 inches is roughly equivalent to 2.5-3.5 bald eagle wingspans.
Overall, 639.84 centimeters or 251.9685 inches is a little over 7 feet in length, which is longer than most humans are tall. It is also significantly shorter than the average length of a sedan car (around 15 feet) or the height of a standard basketball hoop (10 feet).