You can cheat this a bit by using the large designation tool in combination with the small one. IIRC, using the large one first, then using the small one will let you designate the floor under your large designation, which would achieve the affect you are describing, BUT this is the situation where I first found I couldn’t make them dig up too. Try it out and see.
EDIT: Well as usually, I stand corrected. Digging caverns Should give you the info you need. Sorry, dunno how to link directly to the post.