  Studies using the GRB brightness as a distance indicator require a measure of
brightness with a small intrinsic dispersion (close to a standard candle).
There is unfortunately no general agreement on the definition of such a
quantity. We show here that the comparison of the size-frequency curves
obtained with various measures of brightness can be used to select the quantity
which is closer to a standard candle. Our method relies on a few general
assumptions on the burster spatial distribution, namely that nearby bursters
are homogeneously distributed in an Euclidean space with no density or
luminosity evolution. We apply it to 5 measures of GRB brightness in the
Current BATSE Catalog and we find that the GRB size-frequency distribution
depends significantly on the energy window used to measure the GRB brightness.
The influence of the time window being, in comparison, negligible. Our method
suggests that the best distance indicator in this Catalog is the fluence
measured below 100 keV, indicating that GRB luminosities have a smaller
intrinsic dispersion below 100 keV than above.
