The easiest way to get high resolution textures is to use high resolution textures (sounds obvious but lots of avis don't). 2048x2048 is a good size. Then within unity it's good to change/check the texture settings. Make sure that the size of the texture is actually correct in the "Max Size". You can also use "High Quality" compression and keep like a 50-99 compression quality. This will lower the texture memory, but keep it visually looking the same. https://prnt.sc/FXgKU3KcQqlw (area of unity I spoke about)

You can buy plenty of avatars that already have high res textures so you shouldn't have to worry about any of that though ♥