Width & Height

You can set a Layer width and height by checking the width and height checkboxes.
When you check the width or height box, Auto-Layout will set the current value as the constraint value.
The taken value will be either by pixels or by percent, depending on the current toggle state.

Min & Max

You can set the miniumum or maximum value of the width or the height of a Layer

Once set, the layer width or height will never exceed the min/max values, no matter what the Artboard size is.

To keep the layer pinned to the left rather from the right, set the width to % and uncheck Right pin.

Custom Height

In some cases you want to extend the height of an artboard further than the default platform screen sizes, in order to show how the design looks when the user scrolls down.

Auto-Layout will automatically recognize artboards that do not have default screen sizes and will never decrease their height to less than their original height.