Depending on photo content (and quality), I can enlarge an image length and width by 200% or more with minimal quality loss, using methods that vary with the content of the image.  Of course, the edges get a little blurred no matter what, because they are thicker, but I have reduced "jaggy" edges and managed to keep the edge blur to a minimum.  Both of these images have been reduced (by the same proportion) to reduce loading time.