A galaxy cluster's radius (cluster radius) is measured according to various criteria, some based upon the extent of the extra mass density (more generally used for dark matter halos). It is a virial radius. Some that are used:
Distant galaxy clusters are often scaled in relation to the critical density at the redshift of the cluster, because there are galaxy cluster attributes that follow that scale. The commonly-used galaxy cluster radius value R500 indicates the radius that surrounds a volume whose mean density is 500 times the critical density at that redshift. Radii within the cluster can be usefully scaled as R/R500.
Values used, based, e.g., on R500:
Other parameters can be defined similarly.