In 2.5 there will be a feature to define the size of the popup window in the modeler.
That isn't possible yet. In the client the width of pop-ups isn't configurable. It also can't be done with styling, because it's 'hard coded' in the client.
When you give the table in the pop-up a big width with a CSS definition, it will result in a scroll bar in the pop-up, not in a bigger pop-up window.
So a very good question, but at the moment no solution. Please report this feature request in our Partner Portal / MXDN.