Needle sizes range from 2.0mm right up to 25.0mm, and just to make it even more complicated there are three main measurement schemes for needles sizes US, Canadian/UK and Metric.
Would a knitting needle conversion chart make things a bit clearer for you?
The size that is recommended by your knitting pattern will depend on where you live and where the pattern was printed.
If you are in Australia for example, most knitting needles are marked with both the metric size and the UK/Canadian sizes, but if you are buying needles manufacturered in the United States the manufacturer will have used the US sizes.
In other words a 2 in one country is not necessarily the same size as a 2 in another country!
I tend to use metric sizes as no matter where you are 2.0mm is still 2.0mm, and here in Australia, most of the patterns you buy will list both the metric size and the equivalent United Kingdom and/or Canadian size.
As a general guide for selecting your needle size if your yarn is:
If you need to work out what size you should use for to knit a particular item, I recommend that you knit yourself a tension square or two.
A tension square is a piece of knitting that you do before you begin knitting your garment that allows you to measure the tension of your knitting to determine which size of needle you should use.