An HD TV has one native resolution (1920x1080 for a 1080p or 1080i display). If you use anything lower than that, there will be upscaling, which, since it's a TV, will probably be optimized for video. Use the native resolution and scale in your photo software for better results.
Make sure to tweak the TV's display settings. Most that I've seen default to a very strong sharpening filter, which you almost certainly don't want. Ideally, use calibration hardware and software as you would with any monitor.
As for size, I don't think there can be any answer other than "whatever feels comfortable to you?"