a quarter of a wavelength for cb freqs is about 108 inches
you can get antennas this length though they tend to be loaded, (got a coil in them)
the more pysical length thats in the coil the less efficent the antenna is, though for what we need them for it dont matter
when you alter the length, generally you shouldnt have to shorten the whip its self, if its a good design that isnt operating too far from the manufacturers design freq then there should be enuff adjustment with t he grub srew and the hole in the top...
occasionally though and with extra bits of metal next to it, (roll bar) you may have to cut or lengthen the whip
hope that makes sense
dave