I have imported an image with a 1/4 inch scale on paper. Now this is the step I don't understand. Can some body explain this to me?

"Define the scale of the image by moving the end points of the colored line drawn in the image, in such a way that this line matches a known length. Then type the real length of this line in the Length of the drawn line field, and click on Continue."

The other thing I don't understand is "they say don't make your import image to big". But what do they mean by 'too big?