If you scan a stipple drawing at high resolution, you can use Illustrator's Trace feature to convert the image to a vector file. Depending on the image it may work well or not. 800dpi or higher will help insure there is enough image material to capture
details properly. There is a good chance the final file will be even bigger than the stipple image scan, but you can then scan it larger without loosing edge sharpness.
Another option is to use enlarging software, like A.I. Gigapixel to intelligently enlarge a pixel image to poster size. It woks well on some images, not so well on others. I would think it would work pretty well on a stipple drawing.
Another question - is there a way to import a dot and stipple file from Photoshop into illustrator to make it more scalable? I havenít done anything with vector graphics for a long time