I know of limiting the upload size of an object using this method: http://doc.s3.amazonaws.com/proposals/post.html#Limiting_Uploaded_Content
But i would like to know
You may not be able to limit content upload size ex-ante, especially considering POST and Multi-Part uploads. You could use AWS Lambda to create an ex-post solution. You can setup a Lambda function to receive notifications from the S3 bucket, have the function check the object size and have the function delete the object or do some other action.
Here's some documentation on Handling Amazon S3 Events Using the AWS Lambda.