The label on your yarn will indicate the ideal needle size to knit or crochet that yarn. The size on the label refers to the thickness of the needle, not its length.
In Europe, the needle size is given in millimeters. In the United States, the size is a number between 0 and 50. Then in the United Kingdom and Canada, size is a number between 18 and 000.
If you bought some yarn from a brand in a different continent you may like to know how the size translates to a size table you are more familiair with.
Below knitting needle size conversion table will help to find the correct needle size for your project.
Deciding on the lenght of your needle relates to the type of project you like to make. Longer needles are great for knitting bigger projects like sweaters. Shorter needles are great for projects with fewer stitches like scarves.