Scale +and positioning of controls

Interface elements +a user has to select and manipulate should not be smaller than the smallest +average finger pad, that is, no smaller than 1 cm (0.4") in diameter or a +1 cm × 1 cm square.

In the Symbian OS, UI style defines +the target minimum sizes for a UI element considered as finger usable:

7 x 7 mm with 1 mm gaps for index finger usage
8 x 8 mm with 2 mm gaps for thumb usage
List type of components should have minimum of 5 mm line spacing

The width of a finger limits the density of items on screen. If the +items are too close, the user will not be able to choose a single one.

As +the user is more likely to touch higher on the button by mistake than either +side, button and icon sizes can be increased vertically.

Essential information +or features, such as a label, instructions, or sub-controls should never be +placed below an interface element that can be touched, as it may be hidden +by the user's own body.

+ +

With interfaces with traditional input devices, it makes sense to place +targets like menu items on the edges of screens: the user cannot overshoot +the target, as the cursor stops at the edge of the screen.
When using a touch screen, a user seldom drags his finger across the +screen as he would with a cursor. Instead, they most likely lift their finger +and place it on a new target. Users may have difficulties in reaching the +objects located on the edges of the screen, especially if the physical device +has protruding edges around the touch screen panel. Additionally, with some +devices, the screen edges may be less sensitive to register the touch input.