There is nothing special about CB feedlines. Cable length makes no difference if the antenna is tuned and presents a 50 ohm feedpoint impedance.
The 9' foot thing would be if the feedline is acting like an impedance transformer, which is a special case if the antenna is 1/2w.l. and does not have a matching network. A 1/4w.l. of 50 ohm coax would transform a very high impedance as seen on a 1/2w.l. antenna.
However, a good antenna would be built with one (e.g. the Larsen antennas) and the cable length should be the length between your radio and the antenna with enough slack to lay out of the way.
What antenna are you using?
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