I think we have all arrived at the conclusion that you can change the size of the land generated. Its something many wanted. If your only making a 12x12 cell exterior, just do it by hand. Since is suburb it should be mostly flat right? Also keep in mind 12x12 is 144 cells, which is a rather large worldspace, so just make sure you can fill it with stuff. You may want to make it smaller, more like 7x7 as not to overwhelm yourself. But I cant say.
So I reccommend making it by hand.