  We show that galaxy clusters conform to a tight relation between X-ray
isophotal size R_I and emission weighted intracluster medium (ICM) temperature
T_X. The best fit relation for 41 members of an X-ray flux limited cluster
sample is: $log{R_I}= (0.93\pm0.11) log{(<T_X>/6 keV)} -\ (0.08\pm0.01)$;
intrinsic scatter in size about the relation is 15%, and for 30 clusters with
T_X>4 keV the scatter is reduced to 10%. The existence of the size-temperature
(ST) relation indicates the ICM structure is a well behaved function of T_X. We
use an ensemble of gasdynamic simulations to demonstrate that a cluster
population experiencing present epoch growth nonetheless conforms to an ST
relation with scatter similar to that observed; the simulations also exhibit a
tight relation between M_{vir} and T_X, providing the suggestion that a similar
relation holds for observed clusters. We use the scatter in R_I to estimate
limits on the RMS variation in ICM mass fraction \delta f_{ICM} at constant
T_X: \delta f/f_{ICM}<22% (<14% for clusters with T_X>4 keV). It appears that a
mechanism like feedback from galaxy winds, which introduces systematic
structural changes in the ICM, is required to reproduce the observed slope of
the ST relation.
