Current-zero measurements performed during interruption tests have been a powerful tool for measuring the thermal interruption performance of high-voltage gas circuit breakers. This performance is evaluated by estimating the extinction peak of arc voltage (Vp) and/or arc conductance at 200 ns before current zero (G200). However, it is difficult to estimate the interruption performance for actual size circuit breakers. To investigate the size effect, the relationships between Vp and G200 for a small model and actual size model were compared. By introducing an interrupter size factor, we found that the boundary values of Vp and G200, which distinguish the success and failure of interruption, corresponded with each other.