Baby AT is not necessarilly just the "Height" of the board (Length of board perpendicular to slots) it also (maybe more-so) is the width (as measured in parallel to the slots).
In my experience, a FullSized AT board would take a lot of space in width-- maybe twice as much as a standard AT motherboard. I don't think I've seen any AT Boards that are too tall to fit into a Baby AT Case, but they might be too wide. I've never played with baby AT myself, so, I might be wrong about the height issue.
I have a Tyan Tsunami s1830 (Slot1 BX Board)-- it is fullsize AT motherboard. It goes from the back of the case to the front of the case in width (and it's mounted in a full AT tower case). A lot of medium cases the drive bays would interfere with the installation of this board (they'ed block the memory sockets). I know it wouldn't fit into most medium cases and certainly not any mini-cases.