get_bucket_folder_size.py

#!/usr/bin/env python

import boto
conn=boto.connect_s3('api_key','api_secret')
bucket=conn.get_bucket('bucket_name');
keys=bucket.list('path/to/folder')
size=0
for key in keys:
    print "%s:%d" % (key, key.size)
    size+= key.size
print "total: %d GB" % (size / (1024 * 1024 * 1024))