[Avodah] What's a city?

[David Riceman]

In OH 398 it says that a house within 70 and 2/3 amos of a city is part 
of a city.  But I can't find a definition of a city.

If you think of houses as points, and pairs of houses no further than 70 
and 2/3 amos (or possibly double that, for technical reasons I needn't 
go into here) as edges, is a city a connected subgraph? Or is there an 
independent definition of city somewhere which I've missed?

What happens in a town with large lots? For example, a square lot 
containing an acre is 200 by 200 feet.  That implies, if houses are no 
more than 40 feet wide, that they are 160 feet apart.  That it in turn 
would imply that a person living in such a town can walk no more that 
2000 amos in any direction on Shabbos, which seems small to me (the shul 
I attend is 1.1 miles from my house, and 2000 amos is around 0.6 miles; 
luckily I live in a much more densely populated town).

For comparison, in colonial New England, when towns were set up, they 
typically assigned 10 acres per house (with, of course, a much larger 
farm for each household outside of town).

Thanks,

David Riceman