First of: fourstroke engines dont want any restriction,thats BS and if they for some weird reason make more power with a exhaust restriction something is way wrong elsewhere.
Back on topic. have a good look under the car and you will find that you have some space under the backseat that is made to hide mufflers in. magnaflowmuffllers are usualy fairly good. how long of a mufflerbody you want is depending on your skills in building tailpipes ;) you want mufflers with an offset inlet and preferably a center outlet. feel free to ask more if you need any more information.