https://github.com/ToroCraft/NemesisSystem/blob/master/java/net/torocraft/nemesissystem/handlers/SpawnHandler.java#L60

A baby zombie can become a nemesis, but it should always be a baby every time it spawns. An adult zombie nemesis should never spawn as a baby.

Untested.

When saving nemesis information, check if entity instanceof EntityZombie, cast to EntityZombie and check .isChild(). Try using .setChild(boolean) to force whatever zombie we grab on spawn to an adult or baby.