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).