I use firealpaca. Dw, it's for free, has alot of tools and brushes, and it's easy to get into it. I usually use 100x100 for avatars default. Sometimes if you want to use it as a avatar I resize it in another website (I usually use ezgif it's not only for gifs btw.) I never tried procreate before actually, but use anything that you're used to.
It depends. For portrait i usually set it on 1:1 or 1:2 scale on pixel. For example 2000x2000 or 1500x3000 pixel. For avatar or profile picture i recomend 1:1. My standard for head shots are around 2k to 4k pixel because 2k below for me will make it look blurry,at least for semi realism.