No idea what size tyres are best for going low but you need to try and keep the same rolling radius.
If change it too much your speedo (and therfore milometer) will be out.
Try using something like this calculator;
http://www.alloywheels.com/Tyre_CalculatorGoing from the standard 225/40/18, you could get away with 215/40/18s but that's about it.
It helps if you understand the tyre sizes- the first bit is the width of the tyre in mm. The second bit is the sidewall height- it's expressed as a percentage of the width of the tyre.
So 225/40/18 is 225mm accross the tread, the sidewall is 40% of 225mm (90mm) and the rim diameter is 18inches.
If you are sticking with the same size wheel, you need to try and keep the sidewall to roughly the same height (90mm in the above case) to avoid making your speedo read differently.