There are 3 basic elements that first need to be understood regarding adaptive UI's. These are physical size, aspect ratio and resolution.
Physical size is measured diagonally from one corner to another of the display. This has been trending upwards for phones.
The aspect ratio is the ratio of the length of the two sides of the rectangular screen. If the screen was square they would be equal 1:1. 16:9 has become common on phones where it allows the best compromise between keeping a phone narrow enough to grip in your hand while maximising the amount of space that the screen can take up.
This is number of pixels a screen has. This is normally thought of in terms of dots per inch (DPI). This has also been trending upwards and in the next two years 4K (3840 x 2160) with DPI's in excess of 800 are expected. The trend is expected to abruptly stop at this point as the human eye can no longer see any higher.
The next post will show the new adaptable UI issues that emerge as these three elements are combined with how these products are held and used.