There are different ways of knitting in the round and it's just a matter of finding out which one you prefer to use.
If you knit a sweater or cardigan, then you would need a fairly long cable, depending on size of garment - and of course, an even longer cable for knitting a blanket.
When it comes to knitting small items though,you don't need to purchase a circ with a short cable because you can use:
four or five double-pointed needles;
two circular needles;
and then my own particular favourite - using one circular for MAGIC LOOP - funnily enough for babies hats!
Amy has some great videos showing how to use these methods for small items and explains how they're used for adult garments as well.
There are also good videos on YouTube.
All the Best