Sidebar Margin Issues

I have an ad in my sidebar that is being cut off at the right hand side (just beneath the subscription notice at http://newbooksinbrief.com/). When I try to stretch out the margin of the sidebar (which is currently at 22.8%), I’m able able to stretch the margin to 25.4% but no further (beyond this the entire content of the sidebar disappears)–but this isn’t enough to show the entire ad. I know I must have a limitation being imposed from somewhere else but I can’t figure out where (my Firebug doesn’t seem to be helping here).

Any suggestions would be much appreciated. Here’s my amended margin code from the CSS of my child theme (the 22.8% is at the very bottom):

#page {
max-width:1024px;
min-width:1024px;
}

.left-sidebar #primary {
    float: right;
    margin: 0 0 0 -30.4%; /* increased left margin 4% from -26.4% to -30.4% */
    width: 100%;
background-color: #fff;
}
.left-sidebar #content {
    margin: 0 5% 0 38%; /* increased right margin 4% from 34* to 38% */
    width: 58.4%;
}
.left-sidebar #secondary {
    float: left;
    margin-left: 5%;
    margin-right: 0;
    margin-top: 20px;
    width: 22.8%; /* increased sidebar width 4% from 18.8% to 22.8% */
}