One of the best ways to set the scale for our projects is to add a human figure to an image, and by doing that we can show to anyone how big a room, building or architectural structure can be compared with an average human. If you don't have any human figures to use, I found a small collection of images ready to use in architecture at Turbosquid in already with a transparent background. The file is available in PSD and can be downloaded for free.
As a suggestion, you should collect as much human figures as you can. With time your library will grow and shorten the post-processing of your renders.
thank you