Width of Sidebar

I'm trying to add standard sized MPU adverts (300x250px) into the sidebar using the ads section (under Appearance).

The problem is the sidebar isn't wide enough, so I found the style for the sidebar (#main > div > div > .sidebar) and set it to 310px, however it cuts off the right-hand side of the advert. So I made the sidebar wider again, but have the same problem.

I then looked for some padding or margin that might be making a difference, so I set ".wcfAdLocationSidebarTop, .wcfAdLocationSidebarBottom" to have 0 left/right padding (it was 14px). This now displays the full width of the advert, but it looks wrong because there's no left margin, and it really needs it.

I've tried a few other style settings but just can't get it to display correctly. Does anyone have any suggestions??

Many Thanks

Steve

Thanks @Black Rider

I've had another look through things and the changes I made to the style were:

#main > div > div > .sidebar {
	width: 325px;
}




#main > div > div > .sidebar > div {
	width: 314px;
	padding-left: 14px;
}

I then added a div around the advert (within Appearance > Ad) and gave it a negative margin-left of -14px. This way the rest of the items in the sidebar are centered under the MPU.