Enabling Query Store with default values is equivalent to this Transact-SQL statement:

ALTER DATABASE WideWorldImporters 
SET QUERY_STORE = ON    
  ( 
    OPERATION_MODE = READ_WRITE,  
    MAX_STORAGE_SIZE_MB = 100, 
    DATA_FLUSH_INTERVAL_SECONDS = 900, 
    INTERVAL_LENGTH_MINUTES = 60, 
    CLEANUP_POLICY = (STALE_QUERY_THRESHOLD_DAYS = 367), 
    QUERY_CAPTURE_MODE = ALL, 
    SIZE_BASED_CLEANUP_MODE = OFF, 
    MAX_PLANS_PER_QUERY = 200 
  );   

The default configuration is good for small databases or when you want to enable the feature and learn about it with a real query workload. However, for large databases and volatile database workloads you might need to change values for some Query Store options.