It is not possible to display the size of all objects in a bucket in the administration interface.
If you have installed and configured the command line tool s3cmd (see article), you can calculate the size as follows:
$ s3cmd du -H s3://example/ 10G 3 objects s3://example/ $ s3cmd du s3://example/ 10739648901 3 objects s3://example/
Other tools that interact with the S3 API may offer comparable functionality.
Tip: The memory consumption of all your buckets can be checked in the Analytics tab under "Storage bytes".