In Gimp, how can I *throw away* outside transparent pixels

ImageMagick. It's a command-line tool but amazingly powerful and flexible so worth the effort to learn it. For example:

convert -extract 1024x1024+0+0 original.png target.png

Where:

• 1024x1024 is the width and height of the required crop
• +0+0 are x and y offsets into the original image

You can stick dozens of these commands into a .cmd file and run them effortlessly.

Look at the ImageMagick documentation to see there are thousands of options to these commands. A very powerful tool and open source too!

You have to go to Image > Canvas size and unlock the chain icon that keeps the height and the size proportional when you change either of them.