I agree that the runs on a lot of coop pictures seemreally small, but that may be becasue the birds free range most of the day. The run should be three or four times as large as the coop, to support the same amount of chickens. They spend their days outside scratching in the dirt, dustbathing (in surprisingly large holes!), and cavorting about. For the run, the larger the better.
In your warm climate, I think your coop could be smaller than the 4 square feet folks generally use, since they hens will only sleep and lay eggs in there. My coop is really small (coastal CA, mild climate), maybe 30 sq feet for 9 hens. But,  they are never in there except at night (10inches of roost per hen) and to lay (one nest box per four hens). 
You could go as high as 10 hens with a large enough run or allowing them to free range, but trust me when I say you will want more chickens once you have them!