You can’t resize the original highres sprite sheet in photoshop. First you would have to change the sizes in the lua file, too - which is a minor problem.
The big problem are odd sizes and positions of the sprites which will create a mess.
And it’s even simpler than starting Photoshop 
I suggest using a small script file and write both lines for highres and lowres inside. Then you just start TexturePacker and everything is fine 
I am a game developer too - TexturePacker is a side product which I use for my own purposes because “the other tool” gave me a headache handling my near to 700 sprites which I currently organize in 16 sheet at highres and lowres clicking around for hours, resizing sprites and stuff…
I now have a simple bash script which runs in by build toolchain. If I get new assets I drag them into my folders, run the build and the dance is over in less than 15 seconds
Thats faster than even opening one single file dialog…
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