The title says it all.
When placing a canvas or decal image it should show the default width and height (calculated from the pixels and dpi) but have the ability to edit these values, that by default should keep proportions (when editing one the other is updated proportionally), but also have the ability to break that proportion link.
Scale factor, which is what we have now is not really useful, as I have to calculate the scale factor in order to achieve a precise dimension, it's the intended final dimension that is the useful value to edit precisely.