The size of Amzon S3 bucket from S3 console?

This is very easy to do in the new S3 console.

Click on the bucket name, and then select the Management tab and then the Metrics button, as shown here:

You get a simple graph showing the size of the bucket over the last few days, up to 2 weeks, and also (if you scroll down a little) a graph showing the number of objects by day.