I was using 1920x1080, or variations there off. The leaders where 1080 high, width what ever. I did find some images in civ6 files referenced at 1024 height, so that may be their standard setting. Icons are per their size stages, varied between units, leaders & buildings etc, but you can glean these from the DLCs or from my examples in the icon.xlp, if it ends in 256, it is 256x256. All images & icons need to have a transparency/alpha layer.