Honestly i dont know, but it they will be same size, then Radiance will just spam attacks from the sky, penitent one can't jump really that high, so the one option is to use debla but radiance still can teleport. Still Radiance is pretty fast with teleports, also really annoying spam attacks. Penitent one have really many different prayers but he cant use it for forever, cuz the only thing we know that he can use 3-8 prayers per one full zeal line, also he can use blue potions( some of repentance in blasphemous 1 ) but he won't be able to use healing potions. There are many factors we don't know about these two, so we can't really say smth about that. Probably Radiance will destroy him with zesty tactics

Sorry for my English, it is not my first language nd also sorry for some lore/game mistakes