You'll encounter many terms when buying a TV, and it can be confusing. This article is meant to be a simplistic way to consider the purchase, based primarily on two decisions.
Viewing Distance and Field of View
To simplify a complicated question, Basta sa Bahay recommends first focusing on the distance between the viewer and the TV, and second, choosing the field of view the TV will take. We believe that for most cases, the viewing distance is what is pre-determined (we are often putting a TV in a room rather than starting out with a TV and building the room around it). In the past, I lived in an apartment where the TV and the couch had to be positioned flush to opposite walls, and that was a narrow space of less than 2 meters. This was something I could not change.
As for field of view, this is a preference that boils down to what portion of your field of vision your screen will occupy, given that humans have a 200 degree horizontal field of vision. To simplify, we suggest to choose between what are called mixed usage and cinema-like viewing. For mixed (think normal) usage, the TV takes 30 degrees of your field of vision. For cinema-like, the TV takes 40 degrees or more of your horizontal field of vision, and as the name suggests, the experience will be more like how it is in a cinema. Given these two, this is the peg for your TV choice:
Viewing Distance and Field of View Determine Size
Let's explain where this comes from. With the distance you can put between the viewers and the TV, you'll already have a peg for what size (dimensions) your TV should be. This comes from how a bigger TV will mean that the screen will occupy more and more of the field of vision. The peg is to choose a TV size that takes 30 to 40 degrees.
With a 30 degree field of view for the screen, a viewing distance of 1.68 meters pegs your TV at the 40-inch size. If you can put the TV farther away, you'll need a bigger TV to fill the same 30 degrees of your field of vision. Hence, if the TV is 1.83 meters away, you'll need the bigger 43-inch size.
With a 40 degree field of view for the screen, if the TV is 1.3 meters away, you'll need a 43 inch TV. If the TV is 1.9 meters away, you'll need a 60-inch size.
The recommendation comes from two opposing forces. First, you want to get a bigger TV for an immersive experience, and this means that you want at least 20 degrees of that 200 to be filled with the screen. Any screen that is smaller means that most of what you can see with your eyes is non-screen: you're not likely to be able to appreciate what's on the TV. RTings.com gives a good comparison of a 20 degree field of vision versus a 30 degree field of vision. The opposite force comes from how big screens mean that the outer parts now fall in the peripheral part of our field of vision, and for humans, those are the parts that are harder for us to see. This is because light from the peripheries falls on the outer part of the retina that is also the least dense with receptors. TV experts then recommend that the TV takes up 30 to 40 degrees of your field of vision. This means that the screen is not too small nor too big.
Distance and size determines resolution
After size, the next set of options is then resolution. Resolution isn't a complicated concept actually, it really just gives you the number of pixels available. In general, you want more pixels, because each pixel is a point that can be of a different color or brightness, so you'll see more detail the more pixels there are. Often though, resolution is stated as a grid, meaning the number of pixels along the width and along the height. And to make things more complicated, to state it even quicker and partially for marketing purposes, TVs and screens are often described only by the number of pixels along the height, 1080p or FHD means that the unit has 1,080 pixels along its height. Or only by the number of pixels along its width: 4K UHD is actually a round up of its horizontal number of pixels: 3,840. Most often a 4K UHD TV has 3840 × 2160 pixels or 8,294,400 pixels. 8k is also named for the round up of the number of pixels along its horizontal: 7,680 pixels.
Going back, more pixels is generally better: you'll get more detail out of what you are watching. The opposing force however that makes more pixels not any better is that the human eye will fail to distinguish more pixels the more dense they are along our field of vision. This means that at some configuration of distance and screen size, an 8K TV will be the same as a 4K TV: our eyes are just not good enough to appreciate the extra pixels.
Here the rule is simpler, if you opted for the cinema type viewing, meaning you have a bigger TV that takes up 40 degrees or more of your field of vision, then 4K is worth it. You'll appreciate the higher number of pixels because they are spread out in a bigger area, and you can distinguish them more. However, if prefer the 30 degree viewing angle (these are smaller sized TV's), the Full HD (or 1080p) is just as good as the 4K, you may not want to spend the extra money. This is a more complicated chart via rtings.com, but it basically shows how you'll need a bigger TV and a closer distance to appreciate bigger resolutions.
Summary
In summary, to simplify the decision, find out how far you want to put the TV and the viewer, and then second, determine your preference if you want a normal viewing experience or a more immersive cinema-like one. From these choices you'll be able to determine both TV size and resolution.