it's 3/4" O.D. if you're using the stock carb (that's 19mm). To my knowledge the I.D. is either 5/8" or even 1/2" (aluminum molded one is at most 1/2", steel (shiny) is probably 5/8").
My suggestion is make it bigger if you're going to be making one yourself. These engines can barely breathe, you want at least 19mm I.D. if you can. Sadly the stock intake bolt spacing won't allow for such a large tube, so either you need a 40mm bolt spaced intake jug (GT5a, GT2a, GT5-SR, or GT2-SR), or you should taper the pipe. Trust me, 14mm carbs is nowhere near large enough for these engines. High performance 50cc engines use 21mm at least, 80cc road scooters run 19-21mm from the factory. These engines can't breathe, even on the stock intake it makes a pretty noticable difference!
