Our work focused on the widely recognised curvilinear growth response to salinity and the occurrence of succulence (increased ratio of tissue water/dry mass) in euhalophytes. We hypothesized that the curvilinear changes in growth with salinity were largely due to changes in cell size, confirmed by direct measures of epidermal cells and the ratio of tissue water/dry mass, an index of cell size at tissue scale. Two euhalophytes [Salicornia europaea L. and Suaeda maritima subsp. salsa (L.) Soó (syn. Suaeda salsa Pall.) were grown in soil at a range of salinities with water supplied at 40% or 80% field capacity. The salt and water treatments affected plant growth, cell size and tissue hydration. Both species had curvilinear growth responses to the solute potential of the soil solution, with a shoot dry mass optimum and cell size optimum occurring at about −0.6 MPa when watered to the equivalent of 80% field capacity, and about −1.2 MPa when watered to the equivalent of 40% field capacity. Tissue hydration was also affected in a curvilinear manner by the solute potential of the soil solution. For each species, there was a striking linear relationship between tissue hydration and shoot dry mass (P < 0.001), and between tissue hydration and epidermal cell size (P < 0.001). It was concluded that the variation in growth of euhalophytes and their tissue hydration were both caused mostly by the same factor – variation in cell size with salinity.