An app better suited to editing STL scan data or organic models on the iPad is Nomad Sculpt… Its one big failing is that it does not have any means to measure the model… So I would model a rough and simple outline model of the picture frame in terms of the dimensions you want- the back side rabbet inset for the glass and picture and other details easy to model and measure in Shapr… then import that model into Nomad- as your size reference and the object you want to be able to accurately subtract from the scan data of the frame.
Then import your original frame data into Nomad and scale it to fit the objects you modeled in Shapr. This will give you at least one known reference size object to judge the scaling of the frame. You will find that Nomad enables you to stretch or compress the model in any dimension to get it to fit your picture glass, as well as do Boolean operations to cut other model and primitive shapes from the organic model data.
Then you can export STL or OBJ from Nomad to go to print. The one caveat would be to pay attention to the Units you have shapr set to for modeling the glass inset or other size reference objects you will import into Nomad.
When you import the file you saved from Nomad into your printer software- make sure the printer software’s modeling space is set to the same units you used in shapr, and the model SHOULD come in the correct size.
But I would double check by measuring the model in the printer software and scaling it to correct if necessary.