Toilet drains need to be a minimum of 3". The size of the drain lines is dependent on the quantity of fixtures hooked up to them. A shower requires a 2" drain. Some places I'm told require underground drains to be at least 2", but a sink normally doesn't need one that big. You can always go from smaller to bigger as you drain, but never the other way. The drains need slope - nominally, they require 1/4" per foot, so that may be the biggest problem for you depending on the releative elevations of the various buildings.