CB transmitting frequencies is 27Mhz and a full wave at that frequency is roughly 10m so a full wave antenae should be 10m long ish. To shorten this for car use, they put a coil in the bottom and a whip on top, but overall the coil and whip will be, as someone has said, a multiple of 10m or division x2, 5m, 2.5m ish etc.
