I did this pattern multiple times, using different yarns, hook sizes, and stitches each time. The smaller the hook and the stitch, and the thinner the yarn, the smaller the final bear will be.

Even with very small and tight stitches I recommend lining and filling, not just filling. I'm not well versed on this part, but I hear pantyhose/stockings work as lining. I just used what material I had on hand and made little pouches to hold the filling.

Once you pick a stitch, hook, and yarn weight, stick with them. Write them down in your pattern book, as well as the finished bear size, so you can make it again later with adjustments if needed.

You'll need to know:

How to do a Magic Circle start
Invisible Drop
How to sew crochet pieces together
How to sew onto crochet
How to embroider (basics) on crochet stuffies (for the facial features you can use another preferred method for them if you have/know one)

There are videos on each of these topics that can explain them to you so much better than I can.