How can I change sidebar width size?

Hi,
You have to edit your style.css file by replacing :

.sidebar {
background: url(images/sidebar-divider.png) 0 0 repeat-y;
padding: 0 0 0 29px;
margin-top: 30px;
position: relative;
width: 260px;
float: left;
overflow: hidden;
}

by :

.sidebar {
background: url(images/sidebar-divider.png) 0 0 repeat-y;
padding: 0 0 0 29px;
margin-top: 30px;
position: relative;
width: 300px;
float: left;
overflow: hidden;
}

You will however have to change the width of your content block to 595px, otherwise the sidebar will not be able to float left to the content.

Change this :

.content {
float: left;
width: 625px;
padding: 15px 25px 0 10px;
overflow: hidden;
}

to this :

.content {
float: left;
width: 595px;
padding: 15px 25px 0 10px;
overflow: hidden;
}