Linear magnification is a nummerical quantity that determines the scale of the image produced by an object. 
To calculate the linear magnification, you can use 2 formula's:  M = i / o  or  M = Di / Do.
M = i / o: The I stands for the height of the Image. The O stands for the height of the Object. Measure the height of both and you can calculate the linear magnification.
M = Di / DO: The Di stands for Distance from the Image to the lens. The Do stands for Distance from the Object to the lens. Measure the distance from the lens to the object and the distance from the lens to the image. After that you can calculate the linear magnification.