Flexible Width Div Container

Hi guys, I’d like to add a 'Div Container' with a flexible width, of say 20%, to one of my Wordpress pages in which I’d like to place an image.

I believe to do this I need to add a custom ‘style’ via the editor, which apparently will then show up in the drop-down box?

Would anyone be so kind as to help a newbie with an example of the code I’d need to add for this.

Many thanks, Taffy.