Diclofenac is separated by a nanofiltration membrane and the molecular size of diclofenac is needed to prove the mechanism of size exclusion. I did not find the molecular size of diclofenac diethylamine in any reference.
Either look for the unit cells (a, b, and c) from the X-ray crystallographic studies in the literature, or draw the compound in a molecular dynamic software such as Mopac, Gaussian, ... and after getting the equilibrium structure calculate the size form the bond length.