  The utility of a cooperative length scale for describing the dynamics of
glass-forming liquids is shown using literature data on o-terphenyl. Molecular
dynamics and Monte Carlo simulations reveal a distribution of cooperative
fractal events below the caging temperature T_A. Guided by these results, we
show how to extract the size of slow regions in any glass-forming liquid from
probe rotation/diffusion measurements, which agrees quantitatively with 4-D NMR
and grows steadily as temperature is lowered below T_A. We clarify why this
length must also be the size of the largest cooperative events.
