Abstract: Livestock body size parameters are the key indicators to evaluate the production performance of individual and group livestock. Body size parameters can provide an important reference to select excellent breeds. Main phenotypic considerations can also be utilized to select the livestock feed nutrition and the breeding plans. However, the traditional measurement cannot fully meet the high requirements of large-scale production, such as calipers, tape measures, and stick rulers. The manual measurement of the body size of livestock can be time-consuming, labor-intensive, subjective, and invasive at present. More importantly, the invasive measurement is easy to bring stress to livestock, ultimately leading to a detrimental effect on animal welfare. Therefore, it is an ever-increasing demand for the automatic measurement technology of livestock body size in the animal husbandry industry. Most advanced technologies have been applied for the measurement of animal body size, such as digital image processing, machine learning, computer vision, big data, and cloud computing. The automatic measurement technology of livestock body size has achieved better development and progress so far. This review covered the collection and pre-processing of livestock data, the measurement of livestock linear body size, and the measurement of livestock circumferential body size. A full comparison was also made on the general process, commonly-used technology, and research status of automatic measurement for the livestock body size. A specific description was then addressed on the research background and significance, the related concepts, research methods, and application status of automatic measurement for the livestock body size. The first step was the livestock data collection and pre-processing in the automatic measurement of livestock body size. Among them, the visible light camera, depth camera, or LiDAR equipment were used to capture the body surface information of livestock. The digital image processing and computer vision were selected to analyze the livestock images or three-dimensional models. Finally, the dataset was obtained to easily extract the measurement points of livestock bodies. Subsequently, the livestock linear and girth measurements were performed on these datasets. The linear body size mainly included the body length, body width, withers height, chest depth, rump width, and rump height of livestock. Body size parameters were the main reference indicators to reflect the growth status of livestock in the application fields, such as the body condition scoring of dairy cattle, pig body weight estimation, and early diagnosis of livestock diseases. The circumferential body size parameters were also the important reference indicators for the livestock body weight, back fat, meat yield, and skeletal development, including the heart girth, abdominal circumference, and tube circumference of livestock. The circumferential measurement of body size was an indispensable step in the process of livestock breeding. Therefore, the automatic measurement of circumferential body size was developed rapidly in livestock body measurement. New ideas and technologies were emerging in recent years. Three steps were divided, including the points position of body measurement, curve fitting of girth, and size calculation. Some recommendation was also made to promote cost-saving, a high degree of automation, real-time and universality in the field of automatic measurement of livestock body size. The finding can be expected to provide a strong reference for the automatic measurement technology of livestock body size.