Say I have a photo of a part, that I’ve photographed “nicely” from the top-view… and I’ve also placed a reference object (ruler, etc) on the photo to “accurately” size it to scale.
When I import the photo into Shapr3D - it lets me resize it, but the only size I can enter are the actual X/Y dimensions of the photo… rather than say, draw a reference line or circle on the thing to size it (like fusion360 does)…
How is the supposed work correctly?
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
Thanks!
that does help 
much appreciated!!