I don't think that thing on the hidden island in the upper right corner is a tree, its shape is weird and way too big. I has to be the actual shape of the island seen in perspective.
Also, if you look at the official artwork of the Alola region it looks like there's a submarine island in the part that is hidden in clouds in the in-game map. Like there's something there that will come to surface sometime during the story?
Anyway, I made a size comparison of the different islands using Melemele:
View attachment 16115
Akala is almost three times bigger
The big island is five times bigger
The legend island is almost two times bigger
How big do you think this region is? Im having trouble imagining the scale using the official artwork, it looks small to me.