i would really like to have this especially for uploading big size files

You should search this forum, as it has been suggested already. :) I've explained why it's simply not possible with PHP (it is possible with Java or Perl, but we don't want to start relying on outside languages).

Could something like this not be accomplished by the use of Javascript and Php, combined?

The only way it's possible is by patching PHP using custom code, which requires a recompile. The default code base (4x and 5x) doesn't report back upload progress (or speed), so there is no way to know the progress of the upload. For obvious reasons IPS doesn't want to rely on that for a forum feature, it would have to be some sort of mod, and a very hard one to install at that.

Well it's not a needed feature anyway, I was just curious.

You could do this with AJAX and PHP, yes - BUT, you'd need to have full permissions to your tmp directory. I've seen a mod where the AJAX call (during the upload) calls back to the server hitting a php script, which then opens the tmp directory, finds the file being uploaded, reads the size, and reports it back to the AJAX script.

All in all, it requires a special setup to do this (without using Java or Perl), and it's not something we should support.

Besides when most files will be under 2 MB, what's the point really ;)