The difference between HD, FHD, 4K and 8K is related to the resolution of the screen, i.e. the number of pixels that make up the image. More pixels mean more detail and sharpness in the image.
What size TV should I buy?
The size of the TV you should buy depends on the viewing distance and your preferences. Some general guidelines to help you choose the right size depending on the distance are: 1-1.5m -> 32-43'', 1.5-2m -> 43-50'', 2-3m -> 50 -60 '', 3-4 m -> 60-70 '' & 4 m +-> 70''+ .