Press Ctrl+Alt+Shift+S

Click the Manage Styles button near the bottom of the Style pane.

In Manage Styles, click the Set Defaults tab.

Font size should be obvious. For line spacing, look under Paragraph Spacing.
There are two settings. One is the After spacing, which provides 10 point
(default) spacing between paragraphs. The other is Line spacing, which
provides 115% spacing (which isn't  double) between lines within paragraphs.
You can set Line spacing to Single if desired, and you can set After spacing
to 0 points, as well. But, note that the latter will mean that you'd need to
press Enter twice after a paragraph to achieve conventional paragraph
spacing. Experiment and see what works for you. But, this is where you set
these defaults in Word 2007.