What is the length of the focal width x=-1/8y^2

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.