Image layout width of text area

hi, how can i set the width text area of the addon image layout.

Example: http://www.comilfobrugge.be/home

I managed to get the text area the same width as the picture by putting al lot of text.
If i only put a few words, the text area is smaller then the picture, which is not so nice.

So i putted a lot of text. I fixed it.

But when i view the site in tablet or phone view, the text area is again to small...

I can not find any setting to make the background of the text area as big as the image