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This might give you some idea. I stole it from another site.
They're normally rated tread width / overall diameter / rim diameter. Bear in mind that the width of a road tyre is the carcass width, while a slick is measured on tread width, so the equivelant to 185/50-13 would be perhaps 175/515/13.
Also worth noting is that you'll have to change suspension settings and camber at the very least to get the slicks to work properly too