Block-copolymer amphiphiles; have been observed to assemble into vesicles and other morphologies long known for lipids but with remarkably different properties. Coarse-grain molecular dynamics (CG-MD) is used herein to elaborate the structures and properties of diblock copolymer assemblies in water. By varying the hydrophilic/hydrophobic ratio of the copolymer in line with experiment, bilayer, cylindrical and spherical micelle morphologies spontaneously assemble. Varying the molecular weight (MW) with hydrophilic/hydrophobic ratio appropriate to a bilayer yields a hydrophobic core thickness that scales for large MW as a random coil polymer, in agreement with experiment. The extent of hydrophobic-segment overlap in the core increases nonlinearly with MW, indicative of chain entanglements and consistent with the dramatic decrease reported for lateral mobility in polymer vesicles. Calculated trends with MW as well as hydrophilic/hydrophobic ratio thus agree with experiment, demonstrating that CG-MD simulations provide a rational design tool for diblock copolymer assemblies.