How the heck do I explain Dynamax battles to a very casual player who only plays Pokémon GO?
Seriously asking here. I’m not good at explaining things. All I can imagine is confusing them because the Pokémon are already giant sized during raid battles.

Wild Area News 
Gigantamax Eevee will be available in Max Raid Battles until May 25, after which
Gigantamax Meowth will be available until June 2.
Gigantamax Pikachu will be available in Max Raid Battles until 18 May. This pokémon is normally not available in the wild area and was previously an exclusive for having Let's Go Pikachu save data.