Tv's usually have much higher input lag than monitors, if you use your computer for gaming then I would recommend buying a monitor. I hope you have a deep desk so you can position a 50inch screen far enough away to use it comfortably, which in my opinion is the same as using a smaller screen closer, but a smaller similarly spec screen would have better image quality as they have smaller spacing between the pixels, which I find also makes reading small text like what you are reading now easier on the eye. If you want something that multiple people can sit around and watch at a distance then a large tv might be an option, but if you want something you can sit in front of you on a desk then I would go with a monitor.
As you get close to an object it seemingly get larger, it isn't getting larger its just taking up more of your field of vision, eyes have limited angles at which they can process light. Personally i use a 32 inch monitor which I position to take up my field of view, but not to the point where I need to start turning my head, if you need to turn your head then your monitor is too close or too big. If I were to use a larger screen then I would need to move it further away until it fills my field of view, meaning I pay more for a screen that has worse image quality and uses more power for the same field of view.
If you play games and want great close up image quality then I would look at getting a monitor around 32 inches, but it you want something you can watch at a distance and you don't care about input lag, then a large tv would be an option.
Edit:
Before you start looking at screens you should really read up a bit on the different panel technologies, each type of screen comes with its own pros and cons, and understanding them will help you make an informed choice based on your requirements, budget and personal preference.
