Seawater sea sand concrete (SSC) has a wide range of applications in the construction of islands and coastal projects. In marine environments, concrete is prone to cracking, which can significantly affect the durability of structures. In order to ensure the safe service of this new type of concrete in the marine environment, it is crucial to study the fracture mechanical properties of SSC and to determine the fracture parameters reasonably. However, the size effect is inevitably present due to the neglect of material inhomogeneity when using the determined fracture parameters based on traditional linear elastic fracture model. To address this issue, this article aims to determine the size-independent fracture parameters of SSC, utilizing a non-linear fracture theory based on boundary effect model, taking into account the material discontinuity and heterogeneity. In this paper, SSC with the maximum aggregate sizes of 10 and 20 mm were prepared. Three-point bending tests of with heights of 100 and 200 mm were carried out respectively, and the initial crack length-to-beam depth ratios were set from 0.1 to 0.7 in each group. Additionally, fresh water and river sand were used to replace seawater and sea sand in order to prepare ordinary Portland concrete (OPC) as the control group for the experiments. Based on the boundary effect model and by incorporating the average aggregate size of concrete, the size-dependent tensile strength f_t, fracture toughness K_IC of SSC can be obtained analytically from small and medium-sized specimens. Furthermore, the means, upper and lower limits of two fracture parameters with 95% reliability were determined based on the normal distribution analysis. The ultimate load of SSC specimens under any size condition was successfully predicted by using obtained tensile strength. Moreover, the results show that under the same aggregate gradation, compared with ordinary Portland concrete, SSC exhibits a higher proportion of aggregate fracture on the fracture surface and demonstrates higher tensile strength and fracture toughness. With the increase in the maximum aggregate size, the proportion of aggregate fracture in both SSC and OPC decreased. However, the fracture toughness K_IC increased, while f_t decreased. The above model and related results can provide references for the practical engineering design of seawater sea sand sea sand concrete.