Make sure that regions are not in places that are inaccessible in the real world. Try and think about whether there is a good GPS signal in the places that you want to use - the best places for locating regions are in open spaces that don't have high buildings.
Think about how important it is that people don't miss your regions - if you want to make it easy to find then make the region bigger. Also, allow for the way that GPS signals can 'drift' by a few minutes over the course of the day, think about making regions about 20metres across if you want to be really careful. GPS drift will matter if you want a sound to be on a particular landmark. It won't matter so much if you are designing for a field where all the regions will drift together but stay in relation to each other, i.e. they don't move apart; they all move the same amount.