It is your responsibility to ensure that the width/height of content in your Screen is not larger than your Screen. You could use ScrollScreen instead, if you need to scroll down to show more of the screen's content. However, it seems like you would prefer the Panel to scroll, but the height of the Panel is simply too large.
Since I don't know what your screen looks like, it's hard to make specific suggestions, but you might consider looking to setting a layout on the Screen. For instance, if you used VerticalLayout, you could pass VerticalLayoutData with percentHeight = 100 to the Panel, and it would ensure that the Panel would fill the height of the screen (but not more). Another option would be to use AnchorLayout and AnchorLayoutData. If you set the top and bottom properties on the AnchorLayoutData, you can ensure that the Panel is always a certain distance from the top and bottom of the screen.