Pair's database includes over 400 3D models, each of which can be placed into your room within a half-inch of precision. You can insert multiple models and snap screenshots for future reference. The best part? The app is free for users; it's the manufacturers who pay to make their products available to you.
And how does Pair recognize and remember your room's dimensions? Complex visual algorithms place points around the room and track their location, allowing the program to dynamically understand the space even as you move through it. (For the technologically inclined, it's called SLAM: simultaneous localization and mapping).