This script uses the excellent s3cmd to provide a list of all buckets in an account along with their sizes rounded down to the nearest GB MB and Byte. Amazingly there doesnt seem to be an easy way to do this within the s3 web interface.

#!/bin/sh
buckets=`s3cmd -c .yours3cfgfile ls | awk '{FS=" ";print $3}'`
for bucket in $buckets
do
size=`s3cmd -c .yours3cfgfile du "$bucket" |awk '{FS=" ";print $1}'`
sizemb=`expr $size / \( 1024 \* 1024 \)`
sizegb=`expr $sizemb / 1024`
echo "$bucket ${sizegb} GB ${sizemb} MB ${size} bytes"
done

Hope someone finds this useful.