Draw a (possibly temporary) line in Shapr3D in the same plane as the photo – “top view”, etc. Set the line to a convenient size, i.e 12" if that is the length of your reference ruler. Move the photo until one end of the ruler is under one end of the line.
Using the scale tool (on the photo), move the “anchor point” to where the line and the reference point match, then simply increase or decrease the photo size until the other end lines up with the line you drew.
There are several excellent videos on Youtube that show this in detail. Take a look at Import and modify floor plans as DWG files on your iPad | 3D design webinar - YouTube