It is easy in Word 2007… but in Outlook 2007 it is a little different… but here is how!
1. Click “Tools” -> “Options”
2. Select the “Mail Format” tab
3. Click “Stationery and Fonts…”
4. On the Signatures and Stationery dialog box that appears, ensure that you are on the “Personal Stationery” tab
5. Click “Font…”
Then select your favourite font, style, size, colour, and effects… and then click OK