In current eolic research, small Darrieus VAWTs are attracting interest, due to the possibility to produce small machines with low visual impact and almost no noise. Darrieus turbines diffusion, however, is still hampered by the generally poor performance of these machines and particularly by their start-up and transient behavior. In this paper, a critical overview of the main design aspects of a small-size Darrieus is provided. More in details, experimental results and numerical simulations are exploited to stress the effect of the blade design (e.g. airfoil type, pitch angle, blade shape) and the turbine geometry (e.g. Aspect Ratio, solidity) on both the performance and the transient behavior of the machine. The main secondary effects are also analyzed, in order to quantify their relative impact. Finally, some general design criteria and trends for small Darrieus are proposed.