What is your suggestion?

I want to make spiders and bees smaller for exemple, and this size be set for every world I create

I would like this, too. Maybe make a config file or command where entity types can be assigned with a scale type and value and then all entities of those types that spawn will inherit the settings.

I've heard that https://www.curseforge.com/minecraft/mc-mods/vmh can help with this for now, but I second making it inherent to the mod.