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If it's a really large overworld map that is going to involve scrolling, make sure the camera is centered on the player.

Creating an overworld isn't too different from making a regular top-down level. It just seems "different" because you're "zoomed out". 

Like on a regular level, hit detection will be important so your player doesn't walk through mountains/barriers/walls.

As far as distance goes, there needs to be a nice balance. Too far between Town A and Town B is going to get old really fast (especially if fetch quests are involved). Too short between Town A and Town B and people might wonder why you have an overworld map to  begin with.

Those are just a few ideas off the top of my head. Chrono Trigger is excellent! Another classic from that era (my fav) is Final Fantasy 6 (aka Final Fantasy III).