since there was a bit of time before the replies started coming I've started on a similar path using: gm convert -resize '800x200>' inputFile outputFile based on a combination of ImageMagick and GraphicsMagicK CLI documentations
https://www.imagemagick.org/script/command-line-processing.php#geometry
and
http://www.graphicsmagick.org/GraphicsMagick.html#details-resize