The AWS CLI is Awesome

Install (OSX) with brew install awscli and configure with aws configure. Enter credentials + default region (this should be e.g. eu-west-1 not Ireland).

To get the size of a bucket:

aws s3api list-objects --bucket BUCKET_NAME --output json --query "[sum(Contents[].Size), length(Contents[])]"

To download a local dump of a bucket:

aws s3 sync s3://BUCKET_NAME_OR_PATH .