The AWS CLI now supports the –query parameter which takes a JMESPath expressions.
This means you can sum the size values given by
sum(Contents.Size) and count like
This can be be run using the official AWS CLI as below and was introduced in Feb 2014
aws s3api list-objects --bucket BUCKETNAME --output json --query "[sum(Contents.Size), length(Contents)]"
Source: How can I get the size of an Amazon S3 bucket? – Server Fault