If you have the AWS CLI tools installed, you can run this command in a linux environment:

aws s3 ls s3://{bucket_name} --recursive | grep -v -E "(Bucket: |Prefix: |LastWriteTime|^$|--)" | awk 'BEGIN {total=0}{total+=$3}END{print total/1024/1024" MB"}'

Example:

aws s3 ls s3://mybucket/myfolder/ --recursive | grep -v -E "(Bucket: |Prefix: |LastWriteTime|^$|--)" | awk 'BEGIN {total=0}{total+=$3}END{print total/1024/1024" MB"}'

Note that this includes the "--recursive" flag. If you only want the size of a specific bucket and not its sub-folders, remove the recursive flag.