To understand the effect of the finite size of simulation cells on the equilibrium properties of bilayers, an extensive series of glycerolmonoolein bilayer molecular dynamics simulations in which the surface area and system size-were systematically changed have been conducted. Systems ranging from 200 to 1800 lipids were simulated, covering length scales up to 20 nm. The dependence of the surface tension on the area per lipid is shown, although long simulation times were needed (up to 40 ns) to obtain reliable estimates. As the size of simulated patches increases, long wavelength undulatory modes appear with a concomitant/increase in the area compressibility due to coupling of undulation modes to area fluctuations. Both the undulatory intensities and the peristaltic intensities of the-bilayer fluctuations can be fitted in the long wavelength limit to continuum model predictions. The effect of system size on surface tension appears to depend on the stress conditions.