The xwd client in the X11 distributions can be used to select a window or the background. It produces an XWD-format file of the image of that window. The file can be post-processed into something useful or printed with the xpr client and your local printing mechanism.
PostScript output is very small.
The xv program can grab a portion of the X display, manipulate it, and save it in one of the available formats. ImageMagick has similar capabilities. Some X terminals have local screen-dump utilities to write PostScript to a local serial printer.