With the amount you want to drop the car there is no problem going with iether 18s or 20s. It will all depend on the width and offset of the wheel and the width of the tires. If you want to run an 18 most are around 8" wide and if you have an offset around 40ish then you can fit a 215 or 225/40 series tire on it and have no clearence issues at all.
If you want to run 20s then something in that same width and offset range and a 225/30 series tire will give you the same clearence for the fenders.
