In producing crystals via the Czochralski method, achieving equal-diameter growth is essential for ensuring crystal quality. To address the issues of subjectivity and precision limitations associated with manual visual estimation, an online method for detecting the cross-section size of irregular crystal rod grown via the Czochralski method is proposed. First, the crystal growth monitoring video data collected in real time are converted into a continuous sequence of images and preprocessed for denoising and contrast improvement. Subsequently, using weighted pixel values and gradients to determine the crystal rod edges, the similarity of the radius sequence recorded in different directional rays is used as the fitness of the undetermined center, combined with the gray wolf optimization algorithm to search for the cross-sectional center. Finally, the shape of the cross-section is plotted, and the pixel size is converted to physical size combined with camera calibration to estimate the crystal cross-section size. Experiments using Nd∶YVO₄ crystal indicate average errors of 0.30 and 0.10 mm for the major and minor axes, respectively, and an average detection time of 1.07 s, which can facilitate or replace the time-consuming and labor-intensive manual monitoring of crystal growth.