The width of the sidebar on the home page is being set by this CSS:
But as you will see if you change the width of the sidebar -- it will no longer fit on the side -- so you'll also have to change the width of the content section set here:
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.