Cerebral aneurysms come in a wide range of shapes and sizes; they can also evolve over time, presenting significant changes. Large aneurysms are generally thought to be more prone to rupture, but rupture has also been observed in small aneurysms, indicating the presence of additional risk factors. The aim of this study was to assess the effects of the aneurysm's size and wall thickness on its rupture risk, by using fluid-structure interaction simulations. Six patient-specific geometries were studied: four related to the effect of size and two related to the effect of wall thickness. Additional cases in which the aneurysm was removed were included. It was found that thinner walls suffered from significantly greater stresses, whereas an increment in size led, in general, to lower levels of wall shear stress and greater equivalent stress. By removing the aneurysm, the reduction in the time-averaged wall shear stress was 75% at the rupture point. Although the size of an aneurysm has a great impact on its rupture risk, the wall thickness needs to be considered, since with maintenance of its size, an aneurysm can suffer from wall thinning, which can lead to structural failure.