Today’s approaches to modeling the basal area forest trees and stands are based on the models divided into static sub-models describing individual tree and whole-stand variables. This study proposes a general stochastic individual-tree development model with the object to include random forces governing the dynamics of multivariate distribution of the individual-tree size variables. The dynamics of the bivariate probability density function of tree size components (diameter and potentially available area) in a stand is described by mixed effect parameters Gompertz type stochastic differential equations (SDEs) and copula function. This method’s advantages include not having to select many different equations to test, relating the dynamics of the tree size components against the age dimension (time), and taking into account the underlying dependence structure that influences changes in the tree size components. SDE models allow us a better understanding of biological processes driving the dynamics of natural phenomena. We propose a system of the mixed effect parameters SDE to quantify the dynamics of tree size components’ bivariate distribution against the age in a stand with a sigmoid form trend for the mean values of tree size components. The newly derived bivariate probability density function and its marginal univariate, and conditional univariate can be applied to model stand attributes such as the mean tree diameter, basal area, stand basal area, and much more. Therefore, the present study aims to experimentally confirm the effectiveness of using diffusion processes to reconstruct bivariate interactions in tree size components.