You should be able to drag the corner of the window with your mouse. I can resize gVim on Windows 7 this way. The Vim shortcut opens a console window that cannot be resized larger by dragging it with the mouse, so make sure you're running gVim and not Vim.
Alternatively, you can use the keyboard to resize gVim. The exact key sequence to access the Size system menu item depends on the setting of
winaltkeys. If it is set to
yes, you can use Alt+Space to open the system menu. If set to
no, you will need to press and release Alt, and then press Space. After the menu appears, press S to go into resize mode. Use an arrow key to select which side of the window to resize, and then arrow keys to resize that side. This is standard functionality for all (normal) Windows applications, although the
winaltkeys setting is particular to gVim.
Another quick keyboard resizing method in Windows 7 is to hold the Windows key and press the left or right arrow keys. This will make gVim half the size of your monitor on either the left or right side of the screen. This is another standard feature in Windows 7.
Finally, if you are running Vim (the console version), you can still resize the application. Click the system menu (the Vim icon in the upper-left corner of the window, just left of the window title, or press Alt+Space to open the menu) and choose Properties. The Layout tab contains two up-down edit boxes for the Window Size, and increasing them will enlarge your Vim window. If you want to make the window smaller, edit the Screen Buffer sizes. (Basically, you probably want both the Screen Buffer and Window Sizes to match for a console Vim window, and editing either one of them in this manner will automatically update the other.) This is also a standard Windows feature for any console window.