This one is to be consired carefully AND it also depends which template you choose. That sets the width for the main column by percentage from an absolute number. The sidebar colum just fill in the rest by 100%.
Saying that a parameter change in your css depends what your are actually using.
Example for a two third for the main column (standard):
.twothirdcol {
width: 66.7%;
}
You can change it to any number, the sidebar will always follow to fill the rest with 100%
The abolute number for the width of your blog is controlled by:
#wrapper {
width:1020px;
}
Cheers,
Alexander @ http://www.mouseclick.com