I recently edited my Custom CSS file for the WooThemes Daily Edition theme so that my post boxes would fit a 640px wide image, one of the Flickr standard sizes. I haven’t tested this in all browsers, so it may not work, but it looks fine in Firefox and Safari on my Mac. If you’d like to extend your post boxes here’s the css I used. Just paste it into your custom.css file to use.

#header {
width:990px;
}
#page-outer {
width: 990px;
margin:0 auto;
}
.col1{
float: left;
width: 660px;
}
.col2{
float: right;
width: 275px;
}
#nav{width:970px;}
#nav2{width:990px;}
.textwidget {
padding: 0 15px;
}
.post-outer .post-alt { width: 640px;}