What size / length needles for beginner knitting?
It depends on what you are knitting, but you probably need needles that are 60 or 80 cm (24 or 32 inches).

The most commonly used sizes for circular knitting needles are 40, 60, and 80 cm (16, 24, or 32 inches), as measured from tip to tip.  If you buy needles in those lengths, it is likely that you will be able to re-use them for other projects in the future.

60cm and 80cm are very versatile (and commonly used) lengths for circular knitting needles, as they allow you to knit comfortably, both flat and in the round.  For knitting flat, these lengths are ideal because they are a manageable length that is not likely to tangle.  These sizes will also accommodate larger objects in the round (like sweaters), or you can use the magic loop method to knit smaller objects in the round (like hats).