Limit size of HTML/text display box

Hi got Modulobox Premium and it generally works fine. Tried three times to register for premium support via the “Create Personal Token” avenue but it keeps saying token invalid.

Now, to the help I need. I have setup a button to call a display of a box displaying HTML. I have found how to add custom css to change background colour, text colour and width/height of the actual text. But what I want to do is limit the size of the actual lightbox display – at the moment it is full width and does not look like a box. Is this possible and if so can someone point me to a bit of HTML/CSS that I could add?

Many thanks.