Shrink Uploaded file size

Hi every body ... I Searched alot in the web but nothing .......
My Problem is that I want to reduce the actual size of any uploaded file to a specific length.
and I don't want to enforce the User to a specific ContentLength
Is there is anyway of achieving that ... or a better idea fot that problem
ex:-
   Uploaded file size is 1mb (or whatever it's size) --> reduce it to 500kb
   Any Help .... I will be grateful .
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