The measurement of key dimensions of power transformers is an important part of their assembly, manufacturing, transportation, and installation processes. Existing measurement methods are cumbersome and inefficient. Therefore, this paper proposes a visual detection method for key dimensions of 110 kV oil-immersed power transformers. This method utilizes the YOLOv5 object detection algorithm and Grabcut image segmentation algorithm to achieve intelligent detection and segmentation of key components. Then, based on the principle of binocular stereo vision, key dimensions such as the distance between bushings and the maximum cross-sectional dimension of the transformer are measured. This paper establishes a scaled model of the appearance of 110 kV oil-immersed power transformers, and analyzes the effects of factors such as shooting distance and angle on visual inspection of key dimensions of power transformers through experiments. The results indicate that this paper achieved visual detection of key dimensions of transformers under different shooting distances and angles based on scaled model experiments, verifying the effectiveness of this method. This method can provide reference for on-site dimension measurement of 110 kV oil-immersed.