DETROIT — When Marja M. Winters was studying urban planning in graduate school, she learned the art and science of helping cities grow.

Now Ms. Winters, a native of Detroit and the deputy director of the city’s planning and development department, finds herself in an utterly unexpected role, one that no school would have thought to prepare her for: she is sorting out how to help her hometown shrink, by working through difficult decisions that will determine which neighborhoods can be saved and which cannot.

“It was always this notion that the population of the world continues to grow, and more and more people want to live in cities,” Ms. Winters, 33, said about her courses at the University of Michigan. “The reality is very different. Who knew?”

Puzzling through the best way to downsize a city it is not unheard of (it has been considered in Youngstown, Ohio, and Flint, Mich. and even, decades ago, in New York). And Mayor Dave Bing has made it a priority to deal with Detroit’s fast-sinking population and crumbling infrastructure by steering those who remain into fewer neighborhoods, rather than leaving them scattered throughout the 139-square-mile city, whose boundaries made more sense when twice as many people lived here 40 years ago.