  We present a model for describing the general structure of molecular clouds
(MCs) at early evolutionary stages in terms of their mass-size relationship.
Sizes are defined through threshold levels at which equipartitions between
gravitational, turbulent and thermal energy $|W| \sim f(E_{\rm kin} + E_{\rm
th})$ take place, adopting interdependent scaling relations of velocity
dispersion and density and assuming a lognormal density distribution at each
scale. Variations of the equipartition coefficient $1\le f\le 4$ allow for
modelling of star-forming regions at scales within the size range of typical
MCs ($\gtrsim$4 pc). Best fits are obtained for regions with low or no star
formation (Pipe, Polaris) as well for such with star-forming activity but with
nearly lognormal distribution of column density (Rosette). An additional
numerical test of the model suggests its applicability to cloud evolutionary
times prior to the formation of first stars.
