As is described in this previous answer, one way to calculate an estimate of the population size is by using the METAREA variable with the WTSUPP sampling weight variable. This will provide you with weighted population estimates of each metropolitan area able to be identified in the public use files. The Census Bureau warns however that, “Estimates for individual metropolitan areas should be treated with caution. Although estimates for the larger areas such as New York, Los Angeles, and so forth, should be fairly accurate and valid for a multitude of uses, estimates for the smaller metropolitan areas (those with populations under 500,000) should be used with caution because of the relatively large sampling variability associated with these estimates.”