Yes, stock catback sizing is 2.5". The stock tip is 3". You will want an inlet diameter of 2.5" and then an output of whatever size you want. The most common output sizes for this car is 3.5", 3.8", and 4".
Tip brand dousn't really matter, but some of the more expensive ones probably have a higher quality finish, that won't rust or oxidize as quickly if at all. You should give Dimisa tips a look as well.
E90 N54 6AT