There may be other CSS changes you will have to make in order to make this work — it’s often troublesome and complicated to start changing the layout of a page — and that theme has a complex grid system (which I am not familiar with so change as your own risk!). And definitely create and use a Child Theme for any changes — otherwise they will all be lost when the theme is updated. Save a copy too — as you start fooling with it.
Not sure what you want to make smaller in content.
By far the best way to work with CSS is by using a tool such as Firebug.