I don't seem to be able to find a total amount of data I have stored in Amazon S3.
Total per bucket is okay, entire total also, and both would be best.
Go to aws.amazon.com and click Account > Usage Reports
After signing in, select Amazon Simple Storage Service from the Service drop down and then select TimedStorage-ByteHrs from the Usage Types drop down. Select the period of time you want to be reported and use the buttons at the bottom to download the report.
The usage report gives a storage total for each bucket and each day, which is necessary to bill properly for additions and deletions. If all you need is the current total, just select a custom date range that includes only yesterday. If you download in CSV format it should be an easy matter to open the file in a spreadsheet application and sum the storage totals for all the buckets.