Each slice has a width and a height.

You can type in a new width and height, or press Enter or Space to resize the slice using the arrow keys.

Some slice types like Sprites and non-wrapping Text automatically adjust their size when their content changes.

Slices that are set to fill their parent are also automatically sized.

Line slices are a bit special; their size is actually used as the offset between their ends. E.g. a width 10 height 0 Line slice might be drawn from x=50,y=20 to x=60,y=20, a line that's actually 11 pixels long.