Currently, a user has to upload a file in its entirety before they are alerted that the file uploaded is too large.

That is a problem if someone is uploading a large video or similar file and will waste upwards of dozens of minutes on a slow connection before getting the message that their effects were in vain.

There is a new HTML5 File API that can be used and is well supported on current browsers (FF >= 3.6, Chrome >= 19.0, Opera >= 12.0).

Related StackOverflow post: http://stackoverflow.com/questions/11514166/check-file-size-before-upload

Related post: http://i.ndigo.com.br/2012/01/javascipt-checking-the-file-size/