What is the length, in units, of the radius of a sphere whose volume and surface area, in cubic units and square units, respectively, are numerically equal?
The volume of a sphere is $\frac{4}{3}\pi r^3$ and the surface area is $4\pi r^2$, so
\[\frac{4}{3} \pi r^3 = 4 \pi r^2.\]We can divide both sides by $4 \pi r^2$, to get
\[\frac{1}{3} r = 1.\]Therefore, $r = \boxed{3}.$