The main purpose of the present study is to clarify the influences of the dimensions of simulation regions to flow properties such as velocity distributions and drag coefficients. For this purpose, we focus here on subsonic flow of rarefied gas around a sphere. Uniform flows are assumed for outer boundary conditions. The results of simulations show that the values of the drag coefficients have a tendency to increase with the reduction of the simulation regions. The influences of uniform-flow boundary conditions may be negligible and the drag coefficients agree satifactorily with the analytical value of free molecular flow if the simulation regions are sufficiently large. However, since the CPU times become huge roughly in proportion to the square of the number of molecules with dimensions of the simulation regions, we must therefore devise some new techniques concerning outer boundary conditions for the successful application of molecular dynamics methods to outer flow problems, especially three-dimensional flows, which remains to be solved in the future.