The AWS CLI now supports the –query parameter which takes a JMESPath expressions.
This means you can sum the size values given by list-objects using sum(Contents[].Size) and count like length(Contents[]).
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