I've checked several times, and the dimensions of the dipole are spot on.
Are the dimensions your checking the electical dimensions or the physical dimensions ? The optimum aerial will be equal to the wavelength electrically or a dimenion thereof - e.g. a quarter wavelength will work much better than a misc length - some aerial manufacturers will "load" the antenna by adding a coil to make up the electrical length. In a permanent fixture you will adjust by physically cutting the antenna to length based upon SWR value.
If I recall correctly a SWR of 5:1 is totally unacceptable and my meter only ever went up to 5.... Typically if your SWR is incorrect you have an insufficient ground plane (your signal is being reflected back to itself) - try placing some sheet metal under the dipole and grounding the outer brade of the coax from 18mhz dipole to that - for best results again you will want a factor of the wave length on your ground plane for what your trying to balance.
Edited to add an inline amplifier after the SWR meter will distort your readings so balance it before adding an amplifier.