It's the same as with knitting; if you have more stitches per inch/per 10cm or whatever, than the pattern says then those stitches are too small and you need to go up a size or two. If you have less stitches per inch/per 10cm then the stitches are too big and you need to use a smaller hook.
If you are just doing practice pieces and getting the feel of it then the yarn/thread should sit comfortably in the hook. If the hook is too small it will end up splitting the yarn, if it's too big it will be difficult to push the hook through the previous stitches.