When you double the length of the antenna you square the reception (plus or minus) up to half wavelength of the transmission. For CB that means:
If a 1' antenna receives and transmits at 1x
2' = 2x
4' = 4x
8' (steel whip) = 16x
16' (base station) = 256x
This is assuming the same proper install, antenna placement, enough grounding, proper ground planes, and the same radio for all of these points.
If you are on a trail, don't worry too much about it. I run with a 4' or 5' fiberglass antenna depending on how many trees are around.
If I really want my radio to work long distance though then I break out the steel whip, but I won't do that offroad since its too whipy and will break stuff.
The math is a gross approximation, but it's enough to get the idea.