Speckle size plays an important role in speckle-based 3D measurement systems. In real-time systems, frequent speckle size variation may be required as the object goes out of the specified range or the camera aperture has to be adjusted to observe the optimal speckle size on the image plane. In this paper, we present a system that adapts itself to achieve the desired speckle size when the measuring distance is changed. The system uses a motorized stage to vary the distance of the diffuser and the observation plane to change the speckle size which is calculated in real time. The performance of the proposed system is compared with the conventional speckle pattern and the result indicates that the proposed speckle method substantially improves the optimal range of the measurement system.