The definition of molecular cooperativity is discussed. The characteristic length of the glass transition describes the size of this cooperativity. Differential scanning calorimetry (DSC) and heat capacity spectroscopy (HCS) results of a series of poly(n-alkyl methacrylates) (alkyl = methyl, ethyl, propyl, butyl, pentyl, hexyl, and octyl) and a series of statistical copolymers poly(n-butylmethacrylate-stat-styrene) are discussed in terms of molecular cooperativity in the CYP splitting region, where a high-frequency dispersion zone a splits off into the main transition zone CY and a Goldstein Johari process P at lower frequencies. The characteristic length tends to small values of order one monomer diameter in the splitting region for scenarios with an CY relaxation onset. The statements about the size scale of cooperativity are conditional upon certain assumptions leading to the equation used for calculation of this size from HCS and DSC data. The step height of heat capacity (Ac,) and, with less certainty, the square root of the cooperativity volume or number (V;'* or Nil2) are proportional to the temperature distance from the cooperativity onset, T = To,,.