I propose an option in the UNITS panel of the batch processor:
Have the option to set the resolution automatically (for each image) so the long side of the image corresponds to 100 mm.
(100 inches when Unit Inches is selected).
This way the subsequent settings can be seen as a percentage.
Thus width of a frame (canvas) and text size will stay proportional no matter how large/small the actual image.
Not sure what to do about specifying the ppi which will be written into the output file. We probably do not
want to write the one that was used.
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You can use the dimension setting to size the image based on the long/short edge. Don't use a fixed canvas size but use margins so the canvas adapts to the image flexibly.