I believe if you only configure the width or height but keep the other on 0 it will resize it whilst keeping aspect ratio. Could you show your settings?
Otherwise you may get lucky using the oiiotool transcoder if you want a specific output file but also want to apply some image manipulations. E.g. you could resize to 50% like here: Tray Publisher publish ACEScg EXR plates with a sRGB JPEG representation - #6 by BigRoy
The oiiotool arguments can also resize to a fixed width or height whilst keeping aspect ratio. So it could be a workaround if using 0 in settings does not work.