@Manu85 I do not understand what you wrote. I think you want to change the size of the content you are grabbing from a different website.
MMM-SmartWebDisplay creates an iframe and shows a webpage inside that iframe.
Because you are displaying the contents of your link inside an iframe, you are stuck with whatever is coded in that frame. You can’t change what is in it. The web page you chose contents are set to a fixed size by the html code that creates that page.
The width and height in the config.js only affect the size of the iframe that you created with the module. Those values can not affect what is inside the iframe that are hosted on a different server.
Perhaps you can find a different source for your content? If you found something that isn’t set to a fixed size then it may display relative to the size of the iframe you build locally with the module.
This is by design of the www. It prevents hijacking content. If you want to read about it, it’s called same-domain policy.