Take your time. Enjoy the process of learning. You will get better sooner if you relax-- I promise!
Concentrate on the evenness of your stitches at first, not on the size. Just try to stitch with a good even rhythm, making rows of nice, even, same-size stitches. Once you get the hang of it, then you can try to make your stitches smaller.
And you decide what size you like. I know some folks like teensy tiny stitches that are so small they look more like a "." than a "-" !! I like mine to look like this - - - -
Anything worth learning takes time and effort. I hope you can have fun with your lessons and not be critical with yourself. Be kind and breathe! Your stitches will be better with time and practice, and it's well worthwhile.