The minimum lot size in Yuma AZ Is 6,000 sq ft
most lots are 6 to 8,000 sq ft,
43,560 sq ft = 1 acre so 5 lots could fit in to an acre
In a subdivision the number of lots could be 3 per Acre depending on Easements and streets and water retention basin.

It depends on what you are building.
I see most lots in subdivisions of 0.15 acre - 0.25 acre. Larger yards can be as big as 0.5 of an acre but that is usually in more rural areas or in a subdivision with a country feel.

In rural areas, 5 acre tracts in Arkansas may not need "lot split" permission, but that may depend on your county. So, if you had 40 acres, you could just go get a survey and slit 8, 5 acre lots off with out getting the government involved. If you go smaller than 5 acres, they usually require that you go before the city or county to get approval so that they are sure that it fits with their master plan.

I hope that helps. Let me know if there is anything I can do for you.

How Important is Lot Size? - http://www.doityourself.com/stry/lotsize

208.708x208.708 fett = 1 acre
43,560 sq ft = 1 acre
16 sq rods or poles = 1 sq chain
10 sq chains = 1 acre
160 sq rods or poles = 1 acre
640 acres= 1 sq mile
36 sq miles=1 township