At the instance where you add RBCs to a solution of 0.9% NaCl, they will maintain their shape as well as their size, this is simply to indicate an equilibrium. That entails that the solution is isotonic to the RBCs. This must mean that the concentration of solutes on each side must be equal otherwise osmotic pressure would force water either in or out of the membrane. Therefore, one can also safely assume that RBC's also have a solute concentration of 0.9%.
