If I have a pattern for an adult hat that uses 80 stitches and you follow it, you need to get gauge. Using different sized needles or wool or both will change the size of your stitches, but even if you use the same wool and needles, you willl probably not get the same size stitches, because some people knit tightly, others loosely, some in between. So imagine you are a loose knitter.
On the same size needles and same wool I use, same stitch pattern, you will get bigger stitches because you tend to pull your wool more loosely. So my hat with 80 stitches may be the perfect size, but if you cast on 80 stitches and all your stitches are a teeny bit larger than mine because you knit looser, the bigger stitches add up to a bigger hat.
A pattern should tell you what gauge is required to get a certain size, with a sentence saying x stitches and x rows to 10 cm in stocking stitch. You knit a practice square about 13 cm square (with edges of garter stitch to keep it flat). Measure your stitches - how many over 10 cm? If different, you need to adjust your needle size to give you the same size stitches.