Atmospheric rivers (AR) are large and narrow filaments of horizontal water vapor poleward transport. The size of ARs determines how much water vapor is carried from the tropics into higher latitudes, leading to extreme precipitation. Current AR detection and tracking algorithms have resulted in large uncertainty in the estimation of their sizes, with areas varying over several orders of magnitude among different detection methods. We develop and implement five independent methods to reduce the uncertainty and characterize the size of ARs that make landfall over the west coast of North America in the 1980-2017 period. ARs that originate in the northwest Pacific (WP) (100^oE-180^oE) have larger sizes and are more zonally oriented than those from the northeast Pacific (EP) (180^oE-240^oE). ARs become smaller through their lifecycle, mainly due to reductions in their width. They also become more meridionally oriented towards the end of their lifecycle. Overall, the size estimation methods proposed in this work provide a range of AR areas (between 7x10¹¹m² and 10¹³ m²) that is several orders of magnitude narrower than current methods estimation. This methodology can provide statistical constraints in size and geometry for the AR detection and tracking algorithms; and objective insight into AR size studies under climate change scenarios in the future.