[حنيذ]

First les denote the head,tail and the body by the following letters to make the calculations easier:

h = head
t = tail
b = body

h = 6
t = 2h + (1/2)b
b = h + t

thats if by 2x the size, you mean 2 times the size.

t = 2h + (1/2)b
b = h + t

t = 2(6) + (1/2)b
t = 12 + (1/2)b

b = 6 + t
t = b - 6

b - 6 = 12 + (1/2)b
b - (1/2)b = 12 + 6
(1/2)b = 18
b = 36

t = 36 - 6
t = 30

so now you have

Body = 36 inches
Tail = 30 inches
Head = 6 inches

t + h + b = 36 + 30 + 6 = 66 + 6 = 72

The answer is 72 inches