  The results from a systematic study of eleven pulsar wind nebulae with a
torus structure observed with the Chandra X-ray observatory are presented. A
significant observational correlation is found between the radius of the tori,
r, and the spin-down luminosity of the pulsars, Edot. A logarithmic linear fit
between the two parameters yields log r = (0.57 +- 0.22) log Edot -22.3 +- 8.0
with a correlation coefficient of 0.82, where the units of r and Edot are pc
and ergs s^-1, respectively. The value obtained for the Edot dependency of r is
consistent with a square root law, which is theoretically expected. This is the
first observational evidence of this dependency, and provides a useful tool to
estimate the spin-down energies of pulsars without direct detections of
pulsation. Applications of this dependency to some other samples are also
shown.
