The spanning length of a percolation cluster is defined as the difference between the maximum and minimum coordinates of the cluster with respect to some chosen direction. It is statistically related to the number size of the cluster by an exponent that differs from the iriverse dimension that would characterize a compact cluster. This exponent for large percolation clusters in simple cubic lattice sites was studied by the Monte Carlo technique, and results are presented. Previous theoretical treatments of this exponent and its relationship with other critical exponents are discussed.