A range hood should cover as much area above your stove top as possible. It is best to exceed on the sides by 3 inches each, but you cannot have a range hood protruding out over the stove. For a 30 inch stove (back to front) the range hood could be either 21 or 24”. Make sure that you keep the range hood elevation in mind so that the hood extending out does not hit your head or get in the way too much.