Webkit is a web content engine and its not a browser on its own. Anyway, which port of webkit are you referring to? GTK+? QT? EFL?
I just downloaded and compiled WebKit-r70098 (for GTK+) on my Ubuntu box following this instructions. The demo browser (GtkLauncher) that comes with it creates a window of 800x600 (hardcoded in the code) and doesn't have a fullscreen feature.
Also, the pre-compiled binaries for Mac OS X that allows Safari to run on top of Webkit, don't offer native fullscreen capabilities. Currently there's no fullscreen without tweaking Safari configuration files or using 3rd party plugins (there are lots of them).
EDIT:
On Windows, webkit.exe also doesn't have fullscreen capabilities.