what is the optimum rim size to put it on?

I would suggest 16"x8.5" all the way around. You'll have to judge the offset yourself as every vehicle is different and personal perference is drastically different from person to person as well. If you're looking for a staggered setup I would also put 16"x10" on the rear.

Yep, that's pretty damn close. I'm running 225 55 16. Not quite as many tires in that size as 225 50 16, but a lot cheaper than most of the thicker and thinner sidewall choices. $50/corner for decent rubber is a lot nicer than $150/corner

Yeah, it also depends a lot on what you're planning to do with it. 16-inch is the most popular upgrade, and it's the biggest the factory put on. Street racers and corner-carvers usually run 17's (sometimes 18's) because they still want larger tires, but with less sidewall. Less sidewall means less sideways flex in hard corners, but it also means a much stiffer ride overall. If your intention is to be more regular driving than road racing, 16's will offer better tire options, much better looks than 15's, better performance, and still have a good ride quality, as well. 225/55-16 is the factory size for 16-inch wheels and will not mess up your speedometer. That's what I ran on my 'Stang, too.