The screen is the most important component of the tablet computer and its size determines the size of the tablet computer. Usually it is the diameter of the screen plus a couple of centimeters of the "holding part".
For convenient use we recommend approximately 10" tablet computers. 7" screen can be considered small and better suitable for children. If the tablet computer has the screen with the size exceeding 10" it means that the device is quite big and can seem inconvenient for an average user.