If your pattern doesn't state a needle length, then that often depends on you and your knitting style!
Do you like to tuck your needles under your arm? Are you a fan of the magic loop? Think about the circumstances in which you will be knitting and how often you will use the needles after this project.
Shorter straight needles are good for knitting on the go (20cm). If you like to 'tuck' your needle under your arm, a straight needle length of 35cm or 40cm is best.
If you are choosing circular needles to knit something 'in the round' like a sweater or a cowl, you will want to choose a length a little shorter than the circumference of your knitting so that your stitches are not stretched round the needle. For example, if you are knitting a sweater with a chest measurement of 91cm/36 inches, then you would be best to choose an 80cm/32 inch circular needle.
If you like to use circular needles, it's also worth learning the magic loop method and buying longer cabled needles (100cm or more) so you can use them on a greater variety of projects. Magic loop allows you to use a long circular to knit smaller circumferences (eg for sleeves, collars, hats, socks).
Even more information about choosing needles can be found in our Guide.