Coax cable length?

[Silly Wabbit]

I have been told that I need to get a 18' cable for my set up and was also told that it doesn't matter.

Can anyone confirm which is true?

For my 3rd gen and going with;

Cobra 19DXIV 40 Channel Mobile Compact CB Radio
4' Flex Silver Load Series CB Antenna Whip

probably gonna mount it between the hood/fender somewhere. Would love to mount at the rear but I know having a larger surface is better, but don't want to mount on the roof since I will encounter low hanging branches.

Thanks