>>58601258
Arceus' whole map is somewhere between a third and half the size of SV's base game I'd say. It's not an open world but divided into a (smallish) hub town and five areas. You start from the hub town, go out, pick one of the areas from a world map, explore as long as you want then go back to town to report and so on. ZA is tiny compared to PLA, though that's on purpose so it can focus on battles and make the overall pacing much faster. PLA is basically designed the opposite way where you leisurely traverse large areas while catching/studying wild Pokémon and there are very few trainer battles.