If you want to specify a custom size for your needs, enter this in the terminal:
flatpak run org.gnome.TextEditor ~/.local/share/nautilus/scripts/Resize\ Images
Now, in the text editor that opened, on the IMG_SIZES=( section, you can add any size you want, for instance 800x800:
Save and close the text editor. Now you would have the saved size as an option to choose:
Also note:
When resizing an image, the ImageMagick software often tries to maintain the original image’s aspect ratio (the ratio of width to height). This means that when you change the width, the software calculates the corresponding height to maintain consistent proportions. If, after the conversion, you’re not seeing the exact height you chose, it’s likely because the calculated height would result in a distorted image if the original aspect ratio were not maintained.
For instance, if I perform the 800x800 conversion on an image that is originally 3517x1998, I would get a converted image of 800x454 that maintains the original aspect ratio.
Now, if you want the conversion to ignore your image’s aspect ratio and convert it to the exact size you specified, you need to add ! at the end of your size. So on the text editor, add 800x800! instead of 800x800