I worked in land acquisitions for a home builder on the Peninsula before going out on my own. In my experience, most land where residential is currently being built started as commerical or industrial land, and was later rezoned for residential use. As such, most land being sold for residential development comes through commercial brokers (the CB Richard Ellis, BT Commercial, Colliers, etc.). I saw land for sale ranging from a half-acre, on up to 20+ acres. The problem with industrial sites, of course, is the environmental issues, and the fact that most industrial and commercial sellers are not willing to entitle the land for residential prior to selling. This doesn't necessarily help you in your quest for lots to build on, but I hope the info is useful to you nonetheless.