The way I interpreted the question was:
1. Database is in full-recovery mode
2. User foresees bulk-operations (i.e. to be performed sometime in the future)
3. User therefore changes the database to bulk-logged recovery mode
4. User takes a log backup after changing the recovery model
I was unable to find a step that indicates whether bulk-logged operations were actually peformed. As log as bulk-logged operations are not performed, the log file size would not be impacted if the backup was taken immediately after changing the recovery model.
In my humble opinion, the question leaves room for an interpretation that's different than what the actual intention is.