On the left you see a set of scale rules covering all scales. Scale is the difference between the size of the ‘real’ thing and the size you want to model in.
For example, if you want to make a model which is half the size of the real one it is to a scale of 1:2 (1/2); if you want it to be to a scale 50 times smaller than the real thing it is 1:50 (1/50).