There is no hard and fast rule for this. You could try this out.
- On a white piece of paper, using a ruler and a pencil, draw a rectangular box of the dimension.
- Now sign within this box, using a blue or black ball point pen (or whichever color has been suggested).
- Scan this paper and save as pdf or jpeg.
- Now open this file in photoshop or GIMP or any image editor tool you have.
- Use the crop option to cut along the box you had drawn.
- Now save this image.
- Ensure that the border of the box is not visible in the final image you have.
I think this is one of the easy ways to do it.