gschem can export a schematic or symbol page to an image file. To create an image file, use File→Write image… to show the “Write image…” window. This window allows you to select the file to output to, the type of image to create, and (if necessary) the size of the image.
If you select Portable Document Format to output, the “Width x Height” value will not be used. Instead, the file will be sized to fit the visible contents of your schematic or symbol page.