This isn't so much a tutorial but rather a quick tip,
A common misconceptions that plagues beginner pixel artists is the notion that size = realism. That is, that a small sprite doesn't give them enough space to create a character with realistic proportions. However, it all comes down to economy of space.
Obviously, the sprites on the right are the least realistic. However, the answer to making them more realistic isn't to just make them larger, but to use the space differently. The sprite on the far left cuts down on the size of the head as much as possible without sacrificing the ability to be expressive, freeing up the rest of the space for more realistic proportions and more dynamic poses.
Economy of space, in my opinion, ends up being the longest learning process for an artist. When creating the player's first idle pose, I often spend hours tweaking a simple standing pose until I finally feel as though I've stumbled upon the best balance between resolution and detail. The face tends to be the biggest factor.
Never be afraid to spend too much time on the initial pose or facial features. The more time you spend on it, the easier it will be to create poses and expressions that you're satisfied with. Nothing is worse than feeling limited by your own work. I can't tell you how many times I've looked at a sprite's earlier state half an hour later and thought "Damn I'm glad I didn't settle for that"