Increase Sidebar width

I use eleven40(Genesis child theme) theme on my blog. I wish to place a 300×600 ad in the primary sidebar of the blog. But, the size of the sidebar is a little bit smaller than the ad size. I tried to edit it by increasing and decreasing the border margin of the siderbar. Bur it is getting messy when I increased it. It is overlapping the post area.
Check out the Blog here and let me know how can I get extended space in siderbar to place a 300×600 ad.

Thanks.