The place with stones is based off of a commune called 'Carnac' in France. Look it up on google maps, they are placed almost in the exact same spot. Carnac is renowned for it's 'Carnac stones' or rather menhirs (standing stones) surrounding in all sorts of different formations around the area.
As for the few islands to the top left, I believe those are based off the Channel Islands.
Obviously Lumiose City is based off Paris. Not sure about the rest, I have some ideas but they aren't concrete ones.
I guess it doesn't really matter what's based off of what. I just think it's cool to know the back story to it all. Sorry for getting carried away xD
The size of the region being in question, I think it's a fairly large region. A lot of the towns and cities are very spaced out and scattered around the region which makes it not one linear line like Unova was. So I feel like there's more room to travel here.