By definition, mixing in chemical reactors is a multi-scale process since it aims at reducing the length scale on which the chemical components are segregated. Only this way, micro-mixing can be accelerated in order to increase the intensity of mixing. Numerical simulations prove helpful to gain a better understanding of the interplay of transport mechanisms on different length scales, but require means to efficiently extract information about the scales and the quality of mixing. For this purpose, a formula from geometric measure theory is employed to compute both the finest and an integral length scale. Application to a T-shaped microreactor yields interesting information about how the scales depend on the Schmidt number.