We have grown thin opals of self-assembled silica colloids by the well-known vertically controlled drying method. The volume fraction at the start of the growth and the temperature were systematically varied. We have quantitatively characterized the lateral domain sizes by scanning electron microscopy. The sample thickness as a function of position was obtained from Fabry-Pérot fringes measured in optical reflectivity. We observe that the sample thickness strongly increases from top to bottom, independent of temperature, in agreement with a model that we propose. The inhomogeneity in thickness contrasts with earlier reports. The lateral domain shapes of the single-crystal domains are found to vary from irregular near the top to rectangular near the bottom. A surprising observation is that, grosso modo, the lateral domain extents increase linearly with thickness (i.e., thin crystals are small, and thick crystals are large). This behavior agrees qualitatively with results on completely different colloids such as disordered slurries. The consequence of our results for optical applications, including photonic crystals, is that unwanted scattering due to grain boundaries is reduced for large domains that are thick. Conversely, thin crystals will scatter relatively strongly from grain boundaries.