AWS – S3 CLI

How to get Folder size

aws s3 ls --summarize --human-readable --recursive s3://BUCKET_NAME/FOLDER_NAME | grep "Total Objects:"

SAMPLE OUTPUT: 
Total Objects: 13647788

Also this can be done with CLoudWatch:

aws cloudwatch get-metric-statistics --namespace AWS/S3 --start-time 2015-07-15T10:00:00

--end-time 2015-07-31T01:00:00 --period 86400 --statistics Average --region us-east-1

--metric-name BucketSizeBytes --dimensions Name=BucketName,Value=myBucketNameGoesHere

Name=StorageType,Value=StandardStorage

make sure that you StorageType and BucketName.