Are you talking about commissions or about products you want to sell on itch.io?
Personally, I find these questions hard to answer. In principle, I have bought both 2d and 3d assets in the past but at the moment am mostly interested in 2d assets. I will buy individual assets (spritesheet for a single character) or larger collections (texture packs) depending on what I need, and there is no "correct" size for an asset pack. Personally I'd say that with things like tilesets or anything "modular", having more options in there makes the package that more useful. For many other types of asset, selling them individually or in smaller packages works fine.
The price question obviously depends on what you're offering and how much use I can get out of it for my games. But as a hobbyist who currently doesn't even have a tip jar, $20 is kind of the highest amount I can justify spending on any given asset pack at the moment. That would change if I had reasonable expectations of recouping my costs, of course.