A few years ago I read about a decision methodology that NASA uses to reduce the number of options/variables for future missions. Essentially, they make decisions that will remove the largest number of options earlier in the process. This means they can take thousands of variables and chop them down to a handful by making just a few decisions rather than judge each on its own merit. What is the name of this decision-making model?
Generally NASA responds to the decadal surveys put together by the National Academies of Science for each major science branch (i.e., Earth, Planetary, Astrophysics, and Heliophysics). The decadal surveys are constructed by a panel of experts, at the request of NASA, from the respective field and presented as a sort of suggestion for the path the field should take over the next decade. It generally tries to frame the current, big unanswered questions that could be addressed by new/future missions. In those reports, the panels often make suggestions for new missions including size/scale (e.g., MIDEX), goals, and timelines.
There is no law stating that NASA must obey the recommendations of these surveys but there is also no reason to ignore them since they are put together by the very people NASA supports and people supporting NASA. So far as I know, there has never been a decadal survey that was ignored by NASA but there is some "wiggle room" for interpretation. The panels are given a little guidance regarding limitations on (expected) funding and scope of potential future missions but not much else.