As an important parameter to determine the transmission characteristics of a Gaussian beam, the beam waist holds a huge impact in laser technology and imaging systems. Although it is necessary to clearly measure the specific value of the beam waist, the traditional measurement steps are complex and easily introduce error in the measurement process. In this work, we propose an effective method using the in-plane spin splitting (IPSS) generated by the photonic spin Hall effect (PSHE) to precisely estimate the beam waist. We establish a highly sensitive propagation model to describe the relationship between the IPSS shifts and the beam waist of an arbitrary linearly polarized light and then combine with the quantum weak measurement system to amplify the IPSS shifts. We reveal that the IPSS shifts are sensitive to the variation of beam waists when the beam is reflected near the Brewster angle. With the huge amplified IPSS shifts (maximum of 1500 microns), the variation of beam waist can be accurately detected, even by propagation amplification alone. Prospectively, our scheme may provide an effective method for accurately determining the Gaussian beam waist of arbitrary polarization.