The standard equation is 4p(x-h) = (y-k)^2 where |p| = focal distance |4p| = focal width.
x=-1/8y^2
-8x=y^2
4*(-2)(x-0) =( y-0)^2
p = -2, h=0, k=0
focal width = |4*(-2)| = 8
The answer to your question is that the length of the focal width is equal to 8.
