Evaluating the anisotropy of rock and its size effect in spatial variation is of great significance for improving engineering efficiency and stability. For this purpose, an anisotropy index is used to quantify the magnitude of anisotropy in rock. Digital drilling tests are carried out in three directions on six types of rock to investigate the anisotropy of drilling parameters. The rock is subdivided into multiple small units along the depth of drilling, and the effect of size on rock anisotropy based on drilling work was studied. The research findings indicate that the drilling work on six rock types exhibits anisotropy, and the anisotropy sequence for the total drilling work of each rock is as follows: red sandstone > granite > slate > gneiss > sandstone > argillaceous sandstone. As the length of rock units increases, the anisotropy index of drilling work initially exhibits a fluctuating trend, subsequently reaching a state of stability, thereby manifesting a size effect. Furthermore, the representative elementary volume (REV) for each type of rock exhibits distinct variations. The anisotropy indices for drilling work along the depth of the borehole exhibit an initial phase of rapid increase, followed by a convergence towards a value close to 1.0. The anisotropy of rocks demonstrates size dependence, and size effects also exhibit anisotropy. Anisotropy and its size effects are interconnected and inseparable. The reliability of the cited anisotropy index is checked by comparison with two other anisotropy indices. This comparison shows that our method can reliably determine rock anisotropy. In summary, the research outlined in this paper presents a highly efficient and straightforward approach to evaluate the effect of size on rock anisotropy.