The size of animal cells is generally constrained by physiological and developmental factors and rarely scales with body size. Cell size can potentially be influenced by other selective factors that act upon the performance of cell products. In hagfishes, slime gland thread cells (GTC) produce and store threads, proteinaceous fibers that provide strength to their defensive slime. Here, we addressed the scaling of GTC size and thread size with body size by sampling from 20+ species of hagfishes. We found that the maximum GTC length, ranging between 100 – 250 μm, was positively correlated with body size, which varied between 15 – 70 cm in our samples. Despite the size variation, the length to width ratio of GTCs was relatively conserved. Larger GTCs possessed disproportionally thicker threads, the diameter of which increased by four-fold over the range of GTC sizes we examined. A simplified geometric model further suggests that larger hagfishes produce stiffer and stronger slime, with slime threads that are shorter relative to their body length. These results provide insights for understanding the development and evolution of slime threads in hagfishes. This study provides an example of selection on a secreted cell product influencing the evolution of cell size.