Observations of supernova remnants show that large-and small scale structures form at various points in the explosion. We present a case study of structure formation in 3D in a 15 M· supernova for different parameters. We investigate the structure formation and morphology of the Rayleigh-Taylor unstable region. We also propose a method of characterizing the sizes of overdense clumps that can be compared directly with observations. The RT instabilities result in clumps that are overdense by 1-2 orders of magnitude with respect to the ambient gas, have size scales on the level of a few% of the remnant diameter, and are not diffused after the first ∼30 yrs of the remnant evolution, in the absence of a surrounding medium.