First of all i'd like to say that Hachibi's size in this manga has been shown to be rather inconsistent, so his height calculated in this thread is arguable since his size depends on the pictures used for the calculation. I think however that i picked up the ones that seemed the most accurate for me

Hachibi

For accurate calculations, i decided to start calculate his head size in comparison to a character's size that we already know(Naruto). He is 1,66m according to databook

1-Hachibi's head

On this pic like you can see Hachibi's head is 264px when Naruto is 36px. That means Hachibi's head is 7,3333*bigger than Naruto, so 12,17333 meters

2-Hachibi's height

Now knowing the size of Hachibi's head, i made a comparison with his entire body

Infact for more details, i calculated Hachibi's height w/ and w/o his tail

On the above pic like you can notice we have tilting lines, meaning that we'll have to use the distance formula for an exact value of the distance, which isD = sqrt(X^2 + Y^2)

X represent x-axis coordinates of the line

Y represent y-axis coordinates of the line

sqrt represent the square root

D is the distance we are looking for

Like you can see on the picture i already calculated the distance by drawing it with 3different colors representing the calculations of each colored lines.

So Hachibi's head on the picture is roughly 60,876925px, when his body without tails is roughly 235,5525 on the picture and his body with tails being roughly 488,7259 on the picture

With this we easily find out that Hachibi's size without tails is roughly 3,87times bigger than his head, and his size with tails is roughly 8,0281times bigger than his head

Since his head was already rated to be 12,1733 meters, it means we have

Hachibi's size without tails = 47,111 meters(roughly)

Hachibi's size with tails = 97,73 meters(roughly)

Second form Juubi's size calculation

1-Mouth calculation

Like you can see on the picture, Hachibi's body(w/o tails) is 106px when Juubi's mouth in second form is 86px

That means Juubi's mouth is 0,81135 times Hachibi's body(w/o tails)

47,111meters was Hachibi's calculation w/o tails. Which means Juubi's mouth is roughly 38,2236 meters

2-Juubi's size calculation

We see his mouth on this picture is 36px when Juubi's height is roughly 559px

Taking his mouth as a reference tell us Juubi's height in this form is roughly 15,52777 times his mouth

Since his mouth was calculated at 38,2236meters, it means Juubi's height in his second form is roughly 593,6125 meters

Second form Juubi = 593,6125meters

Hope you enjoyed reading