If your pattern doesn't state a needle length, then that really depends on you.
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.
If you're new to knitting, size 8 / 5mm wide needles (25cm long) could be a good idea to learn with, as they are the size most often suggested in knitting patterns and match the most common weight of yarn.
Shorter straight needles are good for knitting on the go (20cm). If you like to use circular needles, it's 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. If you like to 'tuck' your needle under your arm, a needle length of 35cm or 40cm is best.