Step 1: Importing the Image
To import an image, go to the chose the FILE->IMPORT menu option. Make sure you check the "Use as image" option. Sketchup can import several types of image, including JPG, BMP and PNG. Most digital cameras and scanners can provide a picture in such formats.
After you click in open, move the corner of the image to the origin and click the left mouse button once
Without moving the mouse, click the left mouse button again. The image will be outlined in blue.
Step 2: Resising the image
To resize the image, simply enter the value of the known distance. I mean, just push the numbers on keyboard. You can see the numbers while you type in the same place the original measurement were displayed before you start typing.
In the example, I knew that the board was 70mm wide, but a good reference are the distance of a DIL integrated circuit. Just remember that the larger the reference you use, the better precision you get.
After entering the distance, simply push enter. You will be asked if you want to resize your model. "Click YES".