Phase and mixing behavior of dilauroylphosphatidylcholine (DLPC)/distearoylphosphatidylcholine (DSPC) lipid mixtures are studied by molecular dynamics simulations with use of a coarse-grained model over a wide range of concentrations. The results reveal that phase transformations from the fluid to the gel state can be followed over a microsecond time scale. The changes in structure suggest regions of phase coexistence allowing us to outline the entire phase diagram for this lipid mixture using a molecular based model. We show that simulations yield good agreement with the experimental phase diagram. We also address the effect of macroscopic phase separation on the determination of the transition temperature, different leaflet composition, and finite size effects. This study may have implications on lateral membrane organization and the associated processes dependent on these membrane regions on different time and length scales.