A "small auditorium" refers to a compact and cozy space designed to host performances, lectures, or presentations. It is not as large as a typical theater or concert hall, but can accommodate a moderate number of people. Full definition
Small auditoriums with cosy sofas, good selection of films and a very nice restaurant with pleasant atmosphere, delicious meals (although they could work on their veggie options!) (timeout.com)
Films produced by Paul Strand were shown daily in the East Building small auditorium: Manhattan, The Wave, The Plow That Broke the Plains, The Heart of Spain, Native Land, and It's Up to You. (nga.gov)
Below, at the front of a small auditorium, teams of students take turns presenting updates to their peers and instructors about their startups. (fortune.com)