Role of shapes involving smooth surface (circle) and edges (square and triangular-types 1 and 2) for dielectric materials with identical cross section has been investigated for objectives of uniform heating or targeted heating of a representative low lossy dielectric material. The material is incident with microwaves from one side (lateral irradiation) and symmetrically at outer surface (radial irradiation) while the material is exposed to microwaves in free space. Governing equations for microwaves and thermal transport are solved using Galerkin finite element method. Radiation boundary condition (RBC) based on scattering effect has been implemented for microwave propagation. Finite element analysis has been carried out for discretization and detailed implementation on integral form of RBC has been elucidated using finite element method. Average power absorption data for various cross sections have been considered for detailed analysis of uniform volumetric heating or targeted heating effect for various length scales: small samples (Regimes 1 and 2) and large samples (Regimes 3 and 4). Length scales of Regime 2 involving circle-square are optimal for uniform volumetric heating or targeted central heating in the presence of radial irradiation. Triangular materials are very effective for targeted multiple high heating zones at the surface for Regimes 3 and 4 in the presence of lateral irradiation. In general, lateral irradiation provides larger heating effects compared to radial irradiation involving larger length scales (Regimes 3 and 4). © 2021 Elsevier Ltd