Before placing your floor plan image, change your camera view so you are looking at the model from above. This will make it easier to place the image correctly. Camera > Standard Views > Top.
To import the floor plan image, File → Import → navigate to and select the floor plan image. Make sure that you choose the radio button for Image! Click once in the model to place the corner of the image, then drag your mouse to make the image the size you want it. Click once to place the other corners of the image.
Scale your image.
Draw a line on your floor plan using the length guide.
Switch to the tape measure (shortcut key T). Click on one endpoint of the line, then the other end. Type in the correct length length of the line according to the floor plan, then hit enter.
In the pop-up window, confirm that you want to resize your model.