If you're not sure you can always go bigger as there's no downside to that but in most cases it's not necessary. If your car's stock or anywhere near stock the oems or standard NGKs will be fine.

I got the standard NGKs (come in blue) which are just as good if not better than the wires that came on the car from the factory, plus mine have a lifetime warranty.
Although they're slightly bigger than the stock ones they still fit easily into the brackets on the valve cover.