Hello James,
in the Ortho projection tool, Width and Height (outside the Info group) defines the resolution of the rendered projection. When changing one of these parameters, the other parameter and the Ortho pixel size will be recomputed accordingly to preserve the aspect ratio defined by the reconstruction region.
If you are not sure how to define the resolution of an orthographic projection, RealityCapture will calculate the optimal parameters with respect to the properties of a selected model using the ‘Estimate optimal resolution’ function.
Also resolution of an ortho projection depends on the resolution of the texture. If the result of an ortho projection is not good enough, try to unwrap your model with different settings to raise the resolution of the texture and texture again.