Vlad,
For the example I had given in this post you would simply be applying some CSS based on the id of the element. In this case the iframe. If you already have that then the adjustment would be in the stylesheets of the Theme of the site you're working with.
For instance for the default site (Employee Self-Service) it uses the Gray theme by default. To find the stylesheets for the Gray theme click on the module labeled Themes under the Content Management application in the left side nav. This will list any existing themes. Click on the Gray theme. This will list existing stylesheets for that theme.
From here I would create a new stylesheet and name it something like Gray - Custom Styles and make sure to give it bigger number in the order so that it is last in the order. Again going with my previous example I used "myIframe" as the id. To change the width of my iframe element I would put something like this in that new stylesheet:
iframe#myIframe {
width: 75%;
}
Keep in mind this is just one way of doing it. Everything here is just using baseline functionality of ServiceNow's CMS. Depending upon how you built your site there are a few different techniques that could be used.
I hoped that helps.
