I need the sidebar slightly wider

I’ve recently started using a child theme of the twenty twelve theme (Polished, by Tips and Tricks HQ).

Since I’ve kicked off with this theme I’ve noticed that the tall AdSense ad I have in the sidebar (300px X 600px) doesn’t quite fit (it’s as though the sidebar is slightly too narrow).

I’ve looked up how to widen the sidebar but to be honest most of the answers I’ve seen are a bit beyond my coding level (which is pretty basic!).

Can someone tell me, in very simple terms, how to widen the sidebar, or perhaps how to make the ad fit better (for all I know the sidebar might be wide enough but for some reason the ad is just not positioning correctly – it seems to be wide enough for everything else I’ve put in it).

Actually, I’ve just used Inspect Element to take a look at the sidebar and it shows up as:
div#secondary.widget-area 280.953px, so I suppose it’s about 20px too narrow. I could probably change that to 300px, but there’s the whole responsiveness aspect to consider and I don’t really know how to deal with that.

To see the problem, my site is at: https://www.real-memory-improvement.com.

If anyone can help (in very simple terms) I’d very much appreciate it.