Small is nice
Things like forest areas etc don't need to be on the main overworld nor do various islands, they can have their own maps with some sort of link to justify it such as travelling by boat to arrive at the island (a lot of worlds prefer instead to make you swim or have bridges connecting them) or perhaps going into a cave system and then arriving back outside in a forest.
This way the actual travel around the overworld itself to the various access points to these other areas is quick allowing people to easily meet up without having to run for ages.
Big worlds can be fun if there's effective transport (such as 2001 and the train) but then I think the trains themselves were infrequent and the drasine's entertainment value wore off after a while. A more-frequent train would have been a lot more enjoyable - such as every 2-3 minutes!
But then I don't want to support the big overworld argument as it takes too much time to level it all and distracts from content the players will appreciate more - such as using that time to think up and develop quests and other interactive things.