Various aspects of AGN emission are found to vary, suggesting the emission regions are quite small (less than the light travel distance associaated with the variability timescale.
Simple argument for lower limit for the variability timescale for a region of size d is d/c. Simply consider the lag in time between light arriving from the near and far sides of the region. [This particular argument does not include effects of relativistic beaming, which can shorten the apparent variability timescale.]