The longer the glass pack, the more sound it will reduce, and more restriction will be created. There won't be a specific length that will be best unless you are looking for maximum performance which would be best not to have one at all ie. straight pipe.
The internal diameter of the core is what matters. unless your going to be upgrading to huge turbos, 3 inch will be more than enough. depending on how much boost you end up running a 2 1/2 should be sufficient as that is most likely slightly larger than the stock diameter.