Old thread and I cant remember if I posted in here yet but I have tested multiple stinger lengths from 14" down to 6" and inside diameters from 15mm to 22mm on the same bike and I really like 16mm ID with 8" length as a starting point. I chose that ID because based on how ported the exhaust is on a chinagirl your stinger should be 14-17mm ID with a mildly ported jug being close to 16mm. As luck would have it, 3/8" ID is 15.8mm which is close enough and very cheap and common tubing to source locally. The starting length of 8" comes from the generic recommendation of 12x stinger ID which is just under 8", which works out because that puts the stinger just slightly past the seat on many frames which looks good too. You can change where the pipe hits by playing with the ID and length of the stinger, but the ID has a MUCH more dramatic and noticeable effect than the length does unless you get crazy with the length changes.