All exhaust is rated in O.D. Depending on the wall thickness, you can "typically" take off 1/8" for the thickness.
Thus a stock SVT exhaust is roughly 2 3/8" I.D. single and 1 7/8" dual. "Typically" speaking of course.
While a thinner wall does give a greater inner flow path the longevity of the pipe is significantly reduced. Most pipe thickness changes are in the 1/16" range.
