To work out how much liquid a frustum-shaped funnel can hold, which one of these measurements is NOT needed?

Capacity is a volume, $\frac{1}{3}\pi h(R^2+r^2+Rr)$, which uses only the vertical height and the two radii. The slant height belongs to the surface-area formulas $\pi l(R+r)$; substituting $l$ for $h$ in the volume formula is a very common error.