next time your wife logs on...
bring the cursor to the edge of the task bar when it changes into a "double arrow" click on the left mouse button and drag the task bar to the desired height..
if you want to prevent it happening again, right click the bar and check the "Lock the Taskbar"
Windows XP home. If I log on as myself the task bar is about a half inch high. If my wife logs on her task bar is over an inch high. I don't know why it changed but I would like to know how to change it back. Thanks.

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic