In physics, length scale is a particular value of length or distance determined with the precision of one order (or a few orders) of magnitude. Different phenomena occur at different length scales. The typical lengths of all phenomena that occur at the same energy scale are comparable.
The observation that different phenomena should be organized according to the length scale (or, equivalently, the energy scale) is one of the basic ideas of the renormalization group.
For example, the QCD length scale is around 10 - 15 meters, and the "radii" of strongly interacting particles (such as the proton) are roughly comparable. The electroweak length scale is shorter, roughly 10 - 18 meters. The Planck length (Planck scale) is much shorter yet - about 10 - 35 meters.
