Are you referring to scaling a layer to fit the dimensions of a composition, or are you referring to zooming in a view so that the composition fits the size of the viewer?

Shortcuts for scaling a layer to fit the composition are in the "Shortcuts: modifying layer properties" <http://help.adobe.com/en_US/AfterEffects/9.0/WSDFCB87B3-1465-4273-B025-1359940C951D.html> section of After Effects Help on the Web.

Shortcuts for zooming in a view so that the composition fits in the viewer are in the "Shortcuts: views" <http://help.adobe.com/en_US/AfterEffects/9.0/WS8F634346-8CAD-4d79-8987-66AAB92DB1BE.html> section of After Effects Help on the Web.

I found both of these with this Community Help search <http://community.adobe.com/help/search.html?q=shortcut+fit+100%25&lbl=aftereffects _product_adobelr>.