In this work, we designed a visual size-measurement system for micro-miniature optical components. We proposed a dark-field scattering illumination scheme combining dome light and coaxial light to highlight the surface features of the element and used the brightness difference between the subregion and the full image for illumination compensation to eliminate edge ghosting. For the excessively high and uneven local edge brightness, we used a region segmentation-based Otsu algorithm to extract edges. In addition, we used the least square method for the component edge-fitting to calculate the size. Experiment results show that the region segmentation-based Otsu algorithm has an average error of 1.5 and 4.1 μm for measuring length and width, respectively, which reduced by more than 10 μm compared to the Canny edge detection algorithm-based measurement results. The measurement uncertainties of the region segmentation-based Otsu algorithm are 0.5 and 1.1 μm for length and width, respectively. The system is less affected by light, and the accuracy and robustness are higher than the algorithm before improvement.