可以使用SDK，指定文件夹路径列举所有文件夹下对象，获取文件大小，再累加。

以java SDK举例说明，查询桶bucketA下文件夹fileFolder的总大小。

public static void main(String[] args) { 
      String endpoint =""; 
	  String ak = ""; //访问密钥ID,可通过控制台-密钥管理获取
	  String sk = "";//私有访问密钥，可通过控制台-密钥管理获取
	  String bucketName = "bucketA";
	  String fileFolder = "fileFolder";
	  Long fileFolderSize = 0L;

	  AmazonS3ClientBuilder amazonS3ClientBuilder =AmazonS3ClientBuilder.standard(); 
	  AWSCredentials credentials = new BasicAWSCredentials(ak, sk); 
	  ClientConfiguration clientConfiguration = new ClientConfiguration(); 
	  String signingRegion = "cn-north-1";
	  AwsClientBuilder.EndpointConfiguration endpointConfiguration = new AwsClientBuilder.EndpointConfiguration(endpoint, signingRegion); 
	  AmazonS3 s3Client = amazonS3ClientBuilder.withCredentials(new AWSStaticCredentialsProvider(credentials))
	  .withClientConfiguration(clientConfiguration).withEndpointConfiguration(endpointConfiguration).build(); 
	  ListObjectsRequest req = new ListObjectsRequest();
	  req.setBucketName(bucketName);
	  req.setDelimiter("/");
	  req.setPrefix(fileFolder);
	  ObjectListing objectListing=s3Client.listObjects(req);
	  do{
		   for (S3ObjectSummary objectSummary : objectListing.getObjectSummaries()) {
			   		fileFolderSize += objectSummary.getSize();
		        }
		         objectListing=s3Client.listNextBatchOfObjects(objectListing);
		}while (objectListing.isTruncated());
	  System.out.println("文件夹大小为：" + fileFolderSize);
	}