Assessment of Mission Size Trade-offs for NASA's Earth and Space Science Missions addresses fundamental issues of mission architecture in the nation's scientific space program and responds to the FY99 Senate conference report, which requested that NASA commission a study to assess the strengths and weaknesses of small, medium, and large missions. This report evaluates the general strengths and weaknesses of small, medium, and large missions in terms of their potential scientific productivity, responsiveness to evolving opportunities, ability to take advantage of technological progress, and other factors that may be identified during the study; identifies which elements of the SSB and NASA science strategies will require medium or large missions to accomplish high-priority science objectives; and recommends general principles or criteria for evaluating the mix of mission sizes in Earth and space science programs. Assessment of Mission Size Trade-offs for NASA's Earth and Space Science Missions considers not only scientific, technological, and cost trade-offs, but also institutional and structural issues pertaining to the vigor of the research community, government-industry university partnerships, graduate student training, and the like.