Open SpaceClaim

File > SpaceClaim Options > Units > Nanometers > scroll down to the Grid and set it up like this.

Change to the Assembly tab and use the File add button to add the image above.

Drag and stretch the image until it covers 200x200 nm  (I did 100x100 nm). The coordinates of the cursor is shown on the lower left edge of the window.

Now you can sketch over the image. Sketch a closed boundary on 1/4 of the image.

You can sketch only 1/4 of the outline and use Mirror to make the rest.

Now you can pull it into a solid.

Good luck!