Get the biggest freaking tv you can afford. Like 98' movie theatre size.
It all depends on budget. There is no reason to get a 720p if you can afford to get a 1080p tv. If you can afford to get a 1080p tv then why not? If you get bluray players or other HD devices, they could take advantage of the resolution. Some xbox games
are 1080p, so the additional resolution won't go to waste with the 360 either.
I would say 37' 1080p tv is good. Some people will say go bigger and others will say to go smaller if it sports a better picture. Go to you local electronic shops and browse their tvs. Sometimes the blacks look gray and things like that. Picture quality
is also important.
Better of with a 1080p Display, it will be better for the long run. Make sure the TV is a suitable size for you and it runs at 60 HZ. Also go for a good/trusted TV brand such as LG or Samsung and not a cheap one, you get what you pay for.
I own a 60" samsung led-LCD tv, 1080p, 120mhz (2010 model) and it is an awesome looking TV but it's too big when I want to play MP games like COD. For me, it just takes too long to process whats going on. I also own a 32" Sony 3D, LED 240mhz, 1080p (2011
model). Pricey for the size but comes w.all the bells & whistles. I pretty much play all my shooters on the smaller TV & everything else on the 60".