When finding the area of a map the drawing scale is set automatically based on map data. Just zoom in and start drawing.
To set the scale of an image you'll be prompted to draw and enter a 'known length' during image importation. A 'known length' could be any landmark or object on the image you know the length of. It could be a pin head, rular, or doorway.
If you happen to know the scale you need before drawing you can manually set the length of a grid square in the measurement system you chose from the top right corner.