There may be a control within the application you are running Yahoo.
Which browser are you using online to access Yahoo?
In the View menu bar of the open application, there may be a choice
to change the size of the display content in the window itself, check.
Some of these use a keyboard command, should the setting go to
Full Screen, you'd need to know the default or escape combination
of keys to restore the application's window to a standard size. This
kind of inside-application setting is not reflected System Wide.
Since I don't know the browser, (as I have five, each is different)
I can't recommend a keyboard combination to re-size the open
application's view or window setting. They can be non-standard.
But that likely is where you'd have to change it back to normal.
PS: Snow Leopard is Mac OS X 10.6.8.
(Tiger is OS X 10.4.x)
Good luck! 😐