The maximum width that you can increase in template designer in Blogger is 1000 pixels. So,if you want to have 1024*768 size, popular size today, you can't by template designer.
However, in Edit HTML mode, you can.
1. Sign-in your blogger account.
2. Go to Dashboard << Design << Edit HTML
3. Backup / Restore current template (use button in top of the showing page). Whenever you change the HTML template, DO Backup before begin.
4.Find following code. Just use the find function of your browser(in most case, Ctrl+F).
5. You may see portion of codes as follows:
The variable in second line, content.width, specifies total width of your blog. So, you modify the value (orange box), then your blog width increase as much as you want. Note that, the incremental affects to only post area, not side bars. As you see in third and forth line, sidebar width is set as specified value (green boxes). But, posting area width is not. Accordingly, posting area width will increase as the total increased width. Of course, you can increase sidebar width by modifying the values in third and forth line.
Good Luck!
However, in Edit HTML mode, you can.
1. Sign-in your blogger account.
2. Go to Dashboard << Design << Edit HTML
3. Backup / Restore current template (use button in top of the showing page). Whenever you change the HTML template, DO Backup before begin.
4.Find following code. Just use the find function of your browser(in most case, Ctrl+F).
<b:template-skin>
5. You may see portion of codes as follows:
<b:template-skin>
<b:variable default='930px' name='content.width' type='length' value='1000px'/>
<b:variable default='0' name='main.column.left.width' type='length' value='150px'/>
<b:variable default='360px' name='main.column.right.width' type='length' value='270px'/>
<b:variable default='930px' name='content.width' type='length' value='1000px'/>
<b:variable default='0' name='main.column.left.width' type='length' value='150px'/>
<b:variable default='360px' name='main.column.right.width' type='length' value='270px'/>
The variable in second line, content.width, specifies total width of your blog. So, you modify the value (orange box), then your blog width increase as much as you want. Note that, the incremental affects to only post area, not side bars. As you see in third and forth line, sidebar width is set as specified value (green boxes). But, posting area width is not. Accordingly, posting area width will increase as the total increased width. Of course, you can increase sidebar width by modifying the values in third and forth line.
Good Luck!
How to Increase Blog Width Beyond 1000 Pixels in Blogger