The packing cotton picker has become the mainstream cotton-picking equipment due to its advantages of integrated picking and packing and non-stop unloading. Precision packing is an inevitable development trend, and accurate detection of the cotton bale's dimension during the packing process is the first step toward achieving this. Therefore, this paper proposes a geometry-based method to determine the dimension of cotton bale in packing cotton picker. By establishing a mathematical model of the packing process, the relationship between the rotation angle of the rocker arm component and the cotton bale dimension is derived. The reliability and rationality of this method are preliminarily verified through a calculation example. Since the program fails when the rotation angle of the rocker arm component exceeds 55°, the elastic deformation of the packing belt is incorporated for theoretical correction. Finally, the method's accuracy is further validated through field tests, where the absolute error in cotton bale diameter detection is controlled within 50 mm, and the relative error is kept within 2%. This paper lays the foundation for the packing cotton picker to achieve precision packing.