We propose an pan-tilt-zoom (PTZ) tracking method to keep the target object at the center of the image with a predefined size in image. For this purpose, we develop an efficient method for object size estimation using only the tilt sensory data. First we investigate the relational function between the object size in image and the tilt sensory data, and find its approximated function to estimate the object size. Using the approximated function, the object size in each frame is estimated recursively from the known initial size. The proposed method requires smaller amount of computation and shows higher accuracy to keep the size preserving PTZ tracking than the existing state-of-the-art method.