Here are some recommendations for schools on the smaller-side that have strong undergraduate curricula IMO. Most liberal arts colleges have small student bodies and even smaller graduate student populations.
While utterly useless for most purposes, US News’s Rankings ARE helpful for finding schools, whether by student-body size, location etc.
Note: The classifications are based on your stats being competitive for WashU, though definitely research each school in further detail.
Reach: Brown University (Providence, Rhode Island;) Dartmouth College (Hanover, New Hampshire;) Vanderbilt University (Nashville, Tennessee;) Amherst College (Amherst, MA;) Barnard College, if female (New York, New York;) and Pomona College (Claremont, California.)
Match: University of Rochester (Rochester, New York;) Tulane University (New Orleans, Louisiana;) Case Western Reserve University (Cleveland, Ohio;) and Reed College (Portland, Oregon.)
Safeties: Oberlin College (Oberlin, Ohio;) Lewis and Clark College (Portland, Oregon;) Juniata College (Huntingdon, Pennsylvania;) Allegheny College (Meadsville, Pennsylvania;) Trinity University (San Antonio, Texas;) and Southwestern University (Georgetown, Texas.)
Hope that helps! Good luck with admissions!