Generally speaking, PC monitor only goes up to 30" and those are 2560 x 1600 resolution. 1920 x 1200 or 1920 x 1080 monitors typically maxes out at 27". New 27" monitors are starting to be 2560 x 1440 resolution.
If you are looking to use a HDTV for playing games, then you need to realize that there are no "real 120Hz HDTVs". 120Hz PC monitors have 120Hz input and 120Hz output, you use a dual linked DVI cable. DVI operates at 60Hz.
120Hz HDTV (and this applies to 240Hz as well) only have 60Hz input while it can output at 120Hz. HDTVs takes the input signal then creates an interpolated frame in between every two actual frames. This is done to make movies and video playback more smoothly. However, for games it creates input lag because it takes a bit of time to create an interpolated frame. That's why it is recommended when you play games on a HDTV you use "Game Mode" or manually set the HDTV to 60Hz mode.