Fluorescence correlation spectroscopy (FCS) is a mature and powerful technique for measuring diffusion coefficients. In a standard experiment, it measures the spontaneous fluorescence fluctuations arising from a single observation volume defined by confocal optics. However, the study becomes uneasy as soon as the diffusion is impeded by obstacles or specific mechanisms, as it is the case for the cell membrane components in live cells. In this paper, we show that doing FCS measurements at different sizes of observation volumes gives access to the diffusion laws without a priori knowledge of the landscape in which molecules are diffusing. Using this strategy, a measurement of diffusion laws of lipids in monophasic Giant Unilamellar Vesicles and in the plasma membrane of live cells is carried out.