Often, we set simple rules on when to jump up in size or resolution. For example, we start to include the option to customers for 4k displays whenever we are offering a touch display larger than 42 inches. This is a subjective rule, like most things, and it is better consider with some data, but how exactly is that done?
When determining the best size and resolution for a display you should be aware of the angular resolution. Angular resolution is a term used to define the perceived resolution across a given angle. THX, whose name you most likely recognize from your last trip to the theater, recommends an optimal viewing angle of 40 degrees. This means that the best place to sit in a theater for optimal effect is at a point where the right and left edges of the movie screen are 20 degrees left and 20 degrees to the right.
In modern displays, we also understand that our optimal angular resolution is 48 pixels per degree; this is the point when your eye cannot identify individual pixels. Take a look at your monitor.... get closer and your eye can pick out the detail or imperfections created by individual pixels of light; get farther away and things return to focus. By moving farther away, you are increasing the number of pixels per degree in your range of vision. Viewing angle recommendations do vary by manufacturer - making the calculation of pixels per degree a proven method to define not only screen size but also the resolution that best fits your needs.