Molecular dynamics simulations have been used to investigate the effect of system size on the structure of model Langmuir monolayers. Several simulations with 64 molecules were performed in the range 0-185-0-40 nm2 per molecule. In addition, results were obtained for 16 and 256 molecule systems at 0-23, 0-25 and 0-27 nm2 per molecule. The simulations reveal a marked dependence on the number of molecules at 0-27 nm2 per molecule. System size effects play an important role at intermediate densities, mainly due to the competition between the formation of a large tilt angle and the appearance of domains.