In the case of smartphones, the current trend is towards larger displays. Such phones are ideal for web browsing, watching videos or playing games. However, such large displays also have their drawbacks, such as decreased portability and the need to use both hands when calling. Devices with displays between 5 and 7" are sometimes called "phablets" (phone + tablet).
Tablets with a diagonal of 6" to 7" are lightweight and portable, 8" to 10" tablets represent a good compromise between mobility and usability, while large 12" tablets are designed with work in mind.
Diagonal size is one of the key parameters when choosing a laptop. The most common size is 15.6".
Notebook display sizes: