The right size TV depends entirely on preference and priority. If your priority is to have a TV that fits perfectly in the room, then a TV that size is appropriate. If your preference is to have the largest TV possible--even at the cost of sacrificing space, then that TV is the most appropriate. There is no right or wrong answer, you just have to be mindful of placement as it relates to your preference and priority.
Determine the optimal spot for the TV. Identify the room in which the TV will be housed. Select the ideal location for the TV. Measure the space of this location. Decide whether the TV should be wall-mounted or on a stand. If the TV is to be wall-mounted, make sure the wall has studs to screw the mounting brackets into. If there is not a lot of space in the room for a stand, then the TV should be wall mounted, but the advantage of a stand is that the TV can easily be moved later on, without any damage to the wall.
Sit in the area from where the TV will be viewed. Imagine the TV in that space and how big it needs to be in relation to the viewing distance. If the viewing distance is very close (less than 6 feet), a smaller TV can be purchased; if it is very far (more than 15 feet), a much larger TV may be preferable. Determine your budget; a large TV can cost thousands more than a smaller one.
Research TV sizes based on the measurements taken earlier. Determine which size appropriately fits in the space allocated for it. Aesthetically, the TV should not be closer than six inches to any edge, door or adjacent wall. This is simply a design principle used to make sure that art and decor have their ample space. The TV should not distract from the rest of the room but complement the existing decor.