A method, apparatus and product for solving CSP comprising vectors of unknown size. The method comprises generating a structural skeleton tree of a problem description, wherein the structural skeleton tree comprises a node representing a vector of unknown size and a node representing a size of the vector; determining a vector size Constraint Satisfaction Problem (CSP) based on the structural skeleton tree, wherein said determining comprises projecting over-approximated constraints on the size of the vector based on operators used on the vector or elements thereof; solving the vector size CSP to determine the size of the vector; modifying the structural skeleton tree to set the size of the vector and to include nodes for each element in the vector, whereby obtaining a CSP; and solving the CSP.