Hey there, Snakker here, welcome aboard!
Looking good so far, these are good exercises. I agree with Mitsuki, no set brush size as standard, but thinner is probably better for practice on accuracy as smaller mistakes are more visible, thicker brushes are easier to work with
I notice a bit of trouble keeping horizontals, ofcourse you can help yourself with the software for perfect lines, horizontals etc, if your aim is to improve free hand though, and you are working from the shoulder, I find it helps finding a sweetspot angle. Once you are good at one angle, with one line, you can use that one for all your lines - you would only have to rotate canvas and keep doing the same motion. But then, if you want to challenge yourself you can also try getting good at all angles
cheers, and keep at it!