Apartment (28×20) — small dogs, hallway use, crate doors, single-room setups. About the size of a folded bath towel.
Living Room (40×28) — bigger dogs, multi-dog homes, by the back door for the days she can't quite make it. About the size of a yoga mat.
When in doubt, go bigger. The pad does its job whether or not she lands dead-center, and a bigger pad gives her more room to feel okay about using it.