The tendency of atrial or ventricular fibrillation to terminate spontaneously in tissue of finite size is known as the critical mass hypothesis. Previous studies have shown that dynamical instabilities play an important role in creating new wavebreaks that maintain cardiac fibrillation, but its role in self-termination, in relation to tissue size and geometry, is not well-understood. This study used computer simulations of two-dimensional and three-dimensional tissue models to investigate qualitatively how, in relation to tissue size and geometry, dynamical instability affects the spontaneous termination of cardiac fibrillation. The major findings are: 1) Dynamical instability promotes wavebreaks maintaining fibrillation, but it also causes the waves to extinguish facilitating spontaneous termination of fibrillation. The latter effect predominates as dynamical instability increases, so that fibrillation is more likely to self-terminate in a finite size tissue. 2) In two-dimensional tissue, the averaged duration of fibrillation increases exponentially as tissue area increases. In three-dimensional tissue, the average duration of fibrillation decreases initially as tissue thickness increases due to thickness-induced instability, but then increases after a critical thickness is reached. Therefore, in addition to tissue mass and geometry, dynamical instability is an important factor influencing the maintenance of cardiac fibrillation.