You can set the line spacing multiple by using the Spacing pull-down in the ruler.

For very specific line spacing, select the text you want to affect, and then choose Format ‣ Ruler ‣ Spacing… for more options.

Line spacing settings

Reminder that line spacing is a paragraph setting and changes affect the entire paragraph of the selection. Select all text (⌘A) prior to making changes to affect all paragraphs in the document.