I don't understand the column panel 'slot'

I have a form I’m using to ‘pop-up’ a FusionCharts-based graph (which is a form which I’ve made a component with properties, etc). On the form I have a column panel with the card role. Nothing on my parent form is full-width, I want to preserve/rely on the dynamic resizing. However, I’d like this pop-up to be larger, I set the height and width in code and the height value works, but width, beyond. a certain point, does not increase (display wider).

The Column Panel where I have my graph component placed, does not have ‘full width’ option… it simply has the slot as ‘default.’ I don’t understand the slot (not skilled with html, barely understand the javascript I’m manipulating).

Is this something simple I’m missing? Sorry, don’t have a discreet app I can share/clone