The idea that one can derive descriptions of physics at different length scales from one another can be quantified with the renormalization group.
In quantum mechanics the length scale of a given phenomenon is related to its de Broglie wavelength ℓ = ħ/p, where ħ is the reduced Planck constant and p is the momentum that is being probed.
In relativistic mechanics time and length scales are related by the speed of light.
In relativistic quantum mechanics or relativistic quantum field theory, length scales are related to momentum, time and energy scales through the Planck constant and the speed of light.
For instance, in scattering theory, the most common quantity to calculate is a cross section which has units of length squared and is measured in barns.