2.2" is not small. My old Nissan Sentra 1.6SG original comes only 1.8". And that is small. :P hehehe...
I am using 2.5" now though many recomended 3.0". I don;t think there's a need to go oversized. No point. I believed if you strap it on dyno, the improvement is little to none.
About muffler, I am using N1. S-Flow is just sports muffler and not really able to cope with any mid to heavy mods. Let's say if you plan to stay with so-call Stage 1 bolt on mods like increased boost, air filter, TMIC with intercooler piping and fuel regulator, S-Flow can still manage it. Around 240-250hp on crank.
If you wanna go Evo mods like all of us do, TD05, FMIC, downpipe, bigger injector, high boost, cams, remap and more, you need to get N1. Else your car tends to 'choke' on high rev. However, 2.5" is more than enough. At healthy condition, you will see 330-350hp on crank.
If you wanna go extreme like stroker kit, TD06/GT, bigger of everything - intercooler, injector, higher boost, higher cams, uprated internal, then only think about 3.0". Above 400hp on crank...
This is just my personal view... good luck.