Frame does not allow you to simply set a document's line spacing to be single or double-spaced. Instead, you can effectively set the line spacing for a paragraph tag or type (e.g. Body) in one of two ways.
One way to set the line spacing is to move the text cursor into a paragraph and select the Format -> Paragraph -> Designer menu option.
In the Line Spacing field, select the point value which corresponds to single, 1.5, or double line spacing. Then, to set the spacing value for this paragraph only, click on the Apply button. To set the spacing for all paragraphs tagged with the same name, click on the Update All button.
The other way to change the line spacing for an individual paragraph is to move the cursor to that paragraph, click on the button with horizontal lines on it, located under the Format and View menus, and choose the line spacing you desire.