Select "Line Spacing Options" from the menu to customize advanced settings: Pick a value from the "Line spacing" dropdown:
• "Single", "1.5 lines", and "Double" are the same as the numbers above: it just sets spacing to "x" times the line height (based on current font family and text size, in points).
• "At least" lets you set a minimum line spacing in points: this is useful when you have "inline images" inserted throughout your document. In the case of software documentation, including standard 16x16 pixels icons without setting a minimum line spacing would make some lines "higher" than others.
• "Exactly" gives you full control over exact line height, also measured in points.
• With "Multiple", you decide how many times the line's height should be used as vertical space.
Note: the options on the left control "paragraph spacing" - how much space is shown before and/or after each paragraph (by default, in Word 2010, 0 point before and 10 points after).