The effective and computationally efficient task-oriented synthesis of wide reflectarray antennas (RAs) comprising arbitrarily complex unit cells (UCs) is addressed. A multiscale formulation of the reflectarray synthesis problem is proposed where:1) the degrees-of-freedom (DoFs) of the architecture are defined at the microscale in terms of the UC descriptors, while 2) the design objectives are stated at the macroscale as user-defined constraints on the RA radiation features. The arising task-oriented multiscale reflectarray design problem is then dealt with a customized instance of the system-by-design paradigm. A set of numerical examples is shown to illustrate the features of the proposed design approach and to assess, also comparatively, its effectiveness/computational efficiency.