On the default lemmy ui at least, it looks like icons tend to be either ~20x20 when they’re the small circle in the actual post, and 120x120 when they’re in the community view as the 2 main sizes, and 24x24 also shows up in the community list section. Figured I’d mention some sizes in case it helps anyone designing an icon!
Good idea yeah! The 20x20 one will look a little more crunchy than the actual icon would end up, since the actual image file is still higher quality than 20x20 in terms of pixel density, its just the displayed height/width that gets smaller.
The 20x20 one will probably be closer to the same quality as the 120x120 one you posted (probably a bit more blurry), and the 120x120 size will be closer to the full quality one.