One of the most commonly used forms of video lectures is a combination of an instructor's image and accompanying lecture slides as a picture-in-picture. As the image size of the instructor varies significantly across video lectures, and so do the learning outcomes associated with this technology, the influence of the instructor's image size should be evaluated. This study tested the effect of image size of the instructor on learners' experience of social presence, cognitive load, learning performance and satisfaction. In the study, 87 Chinese undergraduates were randomly assigned to view a video lecture with a small image of the instructor, with a medium image size of the instructor or with a large image of the instructor. The results of analyses of variances and analysis of covariance suggested that learners perceived the same level of social presence and cognitive load across video lectures with different image sizes of the instructor; learners who watched the video lecture with the small image of the instructor learned better than those who watched the video lectures with the medium and the large images of the instructors. Furthermore, learners who watched the video lecture with the small image and medium image of the instructor experienced more learning satisfaction. The findings highlight the importance of image size of the instructor in video lecture learning.