This tool helps convert an image you pass in to the three (28x28, 56x56, 112x112) icon sizes required. You can then either download each size individually, or all of them in one. This also works for designing icons for channel point rewards.
It also compresses these icons so that the chances of hitting the filesize limit is minimal.
Note: You'll get best results if you upload a 1:1 image. Any other ratio, and the image will be cropped to fit the ratio.