Period-radius relation for BL Her type stars and the common relation for BL Her and RR Lyr stars based on radii obtained in this work (blue points) and in (Zgirski et al. 2024, orange points). For comparison, we have plotted the relations from the literature: B86 is the empirical relation Burki & Meylan (1986); G17 and G17(M15) are empirical and theoretical relations from Groenewegen & Jurkovic (2017), respectively; and D21 is the theoretical relation from Das et al. (2021).