[Zippy Theme] Make side bar larger for Adsense

I really like this theme but I can’t seem to use the side bar for recommended size google adsense. Even the responsive size ads are cut off by the theme. I need to make the sidebar large enough for the smallest recommended ads which are 300px wide. How can I change it? I know its in the stylesheet but so afraid of messing something up. I just want to change the right info. Please help me. I need ads running when I officially launch my website.