on all of these mufflers i only sell with id in/out, so your od tube slides inside the necks (this is pretty standard for performance mufflers). i offer center/center in both a 4 x 9 and 5 x 11. if you want a louder sound, go 4 x 9, if you want it quieter (and have the room) go 5 x 11. in the 5 x 11, i have a 14" or 18" long body, the longer the body the quieter it is.

This would depend on the sound you were going for.
To measure the diameter you will need to measure across the end of the pipe

That would depend if you are trying to replace it with a stock muffler performance etc. The results will vary by what you have done to the car as well