What are the best 3D scanning mobile apps?
In order to do true 3D scanning you'll need a phone or tablet with a depth scanning technology. Without it, the sizes and/or positions won't be correct.
Google Tango (was Project Tango) delivers 3D sensors in a mobile platform. You can see by this highlight reel Project Tango - Vision Summit 2016 what the technology is trying to accomplish: tracking in a room/indoors, geometry reconstruction, and various augmented reality overlays.
There are limitations like using infrared projector as part of the motion/depth sensing means that it works best indoors.
One phone, the Lenovo Phab 2 Pro is available, and another phone by Asus has been announced for a mid-2017 release.