I can't seem to get the Gmail widget resized to full screen on my SG2. How do I resize it so that it covers the whole screen area?
If you're using the stock launcher, you can't.
If you use an alternative launcher like Nova Launcher or Apex Launcher (one which has support for scrolling & resizable widgets), then you can. Put the gmail widget on your homescreen by long-pressing and selecting the label of choice. THen long-press the widget itself and from the popup menu, choose RESIZE and resize the widget's dimensions as per your liking.
You haven't mentioned what version of Android you're using and Nova/Apex work only on v4+ (ICS). If you have an older gingerbread version, try ADW launcher. Instructions are similar, if not exactly the same.
You may also have to enable the functionality for "scrollable widgets" from within your launcher's settings panel to get the full benefit.