Gail,
That depends on which version of the tree you are using. There is the older Flash version and the newer HTML tree.
For the Flash version, at the bottom of the screen is a Preferences. Click on that to open it. From there you can select how many ancestor and how many descendant generations to show.
For the HTML version, on the left hand side is zoom slider. Above the slider is a gear icon. Click on it to show the controls for how many ancestor and how many descendant generations to show.