The important parameters affecting the impeller performance are the impeller diameter ratio, blade angles and number of blades. The current work describes the design and redesign (Size Optimization) of a 3D centrifugal impeller. Three impeller models with different shape numbers are considered. The initial design is carried out by using the design procedure and empirical equations.  The flow field is obtained by computational flow simulation. Specific work and shaft power are obtained from the simulation and are used to calculate the impeller design efficiency. Thereafter, for a given specific work and flow rate, the impeller size is decreased gradually. As a result, the impeller number of blades and outlet blade angle are recalculated in order to maintain the model design specific work. This is done as before using the standard design procedure. Then, the flow field is simulated to calculate the efficiency. The flow simulation outcomes validate the design and redesign procedure. Accordingly, the efficiency curve against impeller sizes is plotted to study the effect of changing the impeller diameter ratio on the efficiency. Further, the predicted specific work curves for original and optimized impeller are plotted and compared. Finally, for each model, the corresponding peak efficiency size is selected. As expected, the higher diameter ratios are obtained for lower shape numbers. The number of blades and the outlet blade angle increase with decrease in diameter ratio. The lowest diameter ratio is obtained under the conditions that the efficiency be more than 95% of the peak one and the number of blades be under 16.