The Grand Ethiopian Renaissance Dam is a sizable dam on a transboundary river of limited capacity, the Blue Nile River. The economic return from the dam is dependent on how quickly it is filled and operated for optimal power generation. Reservoir sizing upstream on a transboundary river where there is downstream release obligation has constraints. In a transboundary river where the annual retained volume of water to fill the dam is a controversial issue, number of filling years has to be predetermined as a design parameter for the project economic success. The percentage of annual flow that can be retained to fill the reservoir has to be determined prior to site selection and sizing. The economic loss from each year delay in construction and in filling can be determined from investment return of the fund spent on the construction of the dam. One can build a large reservoir on a small river as long as there is no concern in the number of years it will take to fill the dam and no concern for downstream releases. That is why the number of years to fill the dam has to be a design criterion in sizing the reservoir, selecting the site, and operation plan development accommodating downstream demands. Size of the dam is related to number of filling years for the average river flow rate. Downstream countries like a slow rate of dam filling and guaranteed releases, especially during droughts to maintain the status quo. The reservoir has been filling from 2020 through 2023 reaching 40 bcm storage and water level of 620 m asl by fourth filling in September 2023. Two turbines are generating power.