This paper presents the results obtained by multiphysics (electromagnetic and thermal) numerical simulations which show the influence of the size in two different spheres that are irradiated by an electromagnetic plane wave. Each sphere is constituted by a series of concentric layers that emulate the tissues in the head. A 10 mW/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> plane wave is used as source, and the frequency is setup to 1.8 GHz. The power absorbed per unit volume by the tissues (W/m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> ) is calculated by using the 3-D Finite Difference Time Domain (FDTD) method. Then, temperature inside the spheres is calculated by using a Finite Differences (FD) formulation of the heat diffusion equation. Two different ways of calculating the power-loss density are presented, and validated with analytical results. It is observed that the increase of temperature inside a 66 mm sphere is higher than inside a 100 mm sphere. However, the power absorbed per unit volume in the brain zone is six times higher in the 100 mm sphere.