Monte Carlo generation of polymer chains allows evaluation of the steric partition coefficient, K, for any pore shape and any chain flexibility. This in turn enables the chromatographic radius of macromolecules, R(c), to be defined as the radius of a sphere with the same K value and allows the ratio R(c)/R(eta) to be studied. R(eta) is defined as the radius of the sphere with the same product [eta]M, where M is the molar mass and [eta] the intrinsic viscosity. R(c) is shown to depend on the pore geometry. For a given pore geometry, R(c)/R(eta) is not strictly independent of the flexibility and the relative thickness of the macromolecule. Experimentally, for flexible polymers, the so-called universal calibration is often found to work well. However, simulations show that the universal calibration is not strictly valid and deviations are expected in particular when the global flexibility of standard and sample are different.