auto resize full size images

Hello,

I get how to resize my thumbails, medium and large image sizes.

I don’t want users to be able to upload massive poster-sized digital photos.

I don’t want them to get an error either, though. If they upload some 10 megapixel image, I want the server resize that to about 1200×900 and only store that on the server as the full size image.

How do i do this? Thanks.