This popped up on CN recently so with a few cloudy days ahead I thought I would give it a go. I seem to have it working now with GIMP.
https://solarchatforum.com/viewtopic.php?f=4&t=29125
I applied it to one of my images from last week and it gives you an idea of size/scale of Active Regions/Sunspots/Proms compared to the Earth and Jupiter. That big prom is about 35000Km high and the Sunspot in AR2786 would easily swallow the Earth!
The 2nd image is of the actual downloadable pdf ruler template (I had a friend convert it from French - thanks Pat!) that you can import to GIMP and Photoshop as a layer.
If you just want the pdf (its 330kb) I can send it to you, just PM me.
Hope this is useful.