Definitely thicker clumps. Messy single-strand pixelling pretty much never looks good.

There is nothing different about pixelling portraits compared to pixelling anything else, so it's not surprising that there aren't tutorials specifically about this. Try looking up tutorials on drawing portraits, they'll have advice you can use about placing features and lighting, which you can then translate into the pixel medium.
I found this one useful when I was starting out. It doesn't talk about feature placement, instead it focuses on defining those features with light. For feature placement, there are thousands of tutorials that all say basically the same stuff.

You currently have some construction issues, which I think you should address before you get into pixel-level work. Her eyes are different sizes (beyond what perspective could explain), her face outline and facial features are aligned in different directions, her neck is connected a bit weird, and I can't make sense of her breasts. Try expanding your canvas so you can draw in more of her body to make sure it makes sense, then crop afterwards.
The tutorial I linked above has some book recommendations. Some of those books are available for free online.