Increase Sidebar Width on TwentyTwelve

Hi There,

I am currently using a Customized version of TwentyTwelve on my blog jjbegonia.com. I would like to install an Ad that is 300X250. As the Ad is getting cut-off with my current settings, is there a code I can use to increase the sidebar width? I am very new to CSS, so any and all help is much appreciated. Thanks so much!

Best,
Carlynn
jjbegonia.com