How to limit image size for the entire website ? But without editing the post (css/plugin?!)

Quick fix via CSS would be using max-width property to style images inside posts. Unfortunately it is very unreliable in Internet Explorer.

.entry img { max-width:500px !important; }

Also in your specific case image has size defined with inline style and it interferes.

Plugin method would involve scanning and changing posts content on the fly, which would probably bring in performance issues, etc.

In my opinion it is better to fix such things consistently on content level. If you have no authority to edit posts show the issue to person who has.