Change the width and height html attributes to old posts

Hello,

How do I change the width and height of images already inserted into old posts? I need to make them bigger but wordpress writes these in the html so it’s not possible with plugins such as vipers.

I could do it with css if they were to be smaller. But still it wouldn’t be a clean solution.

Is there any other way? I have 3000 posts into my blog and can’t do it manually!

Is it possible that wordpress writes these attributes into the html? Not even facebook does it into notes…

Thank you very much

Simon