The occurrence of glass transition is believed to be associated to cooperative motion with a growing length scale with decreasing temperature. We provide a route to calculate the size of cooperatively rearranging regions (CRR) of glass-forming polymers combining the Adam-Gibbs theory of the glass transition with the self-concentration concept. To do so we explore the dynamics of glass-forming polymers in different environments. The material specific parameter alpha connecting the size of the CRR to the configurational entropy is obtained in this way. Thereby, the size of CRR can be precisely quantified in absolute values. This size results to be in the range 1-3nm at the glass transition temperature depending on the glass-forming polymer.