Here is an example of code that will expand the main column in the Grisaille theme by 200 pixels:
#primary-content {
width: 825px;
}
.home-post {
width: 805px;
}
#header > a,
#top-menu,
#canvas {
display: block;
width: 1160px;
}
#header > a img {
display: none;
}
#header > a {
text-align: center;
background: url('http://nikkoazan.files.wordpress.com/2012/10/header1.jpg') repeat-x;
height: 200px;
}
You can add it on the Appearance → Custom Design → CSS page, and you should also increase the "Content Width" setting on the right by 200 pixels as well (so change it to 790).
Note that the images may not adjust automatically and you may need to re-insert some of the past images to get them to show at the larger size. Whether or not you need to do this will depend on what settings were used at the time each image was inserted. I think they should expand automatically if you had inserted the images at "full size" when you created the existing posts, but you would need to save the changes to test it out.