I've heard that you need 5,000 people for a 30,000-50,000 sq ft. grocery store. Does anyone know anything about this?
This is just slightly higher than the typical suburban density of ~3,000-4,000 people/sq. mi.
Does this mean that if density is increased slightly - from 4-5 to 7-8 units/acre, that a full service grocery store could be located every square mile? This would be similar to the density of most streetcar suburbs which had stores within walking distance. Increasing density like this in low density suburbs could easily be done by allowing secondary dwellings in the backyard or by allowing people to convert their garages into an apartment.
Obviously the number of people needed to support a store will vary depending on demographics (average income). There should be an average though that will apply to the majority.