Both height and width can optionally be set to produce a more stable tabbed area. In the default scenario, the tab content height flows to match the content it contains but that can cause issues in some pixel perfect skin designs. By adding a height and/or width limit to the module, you are guaranteed that the module will only take up a certain area of the screen real estate.
Example 1 - shows setting the width, leaving the height fluid
Example 2 - shows setting the height, leaving the width to the maximum of the pane area
Example 3 - shows a combination of both, notice the scroll bar in the content area allowing all of the content to still be seen