Text (block) width inside editor

How can I change the width of the text inside the editor field?

My theme adapts the width according to the pixels of the device – and it seems to me that tinymce changes this width according to the lowest value.

So the editor itself is always full width on my screen – but the text in the block itself is way too narrow. I would like to set it to fullwidth or alternatively 1900 pixels. How can I do that?

I found something about texarea_rows but don’t know if I’m really looking at the right thing.