How to change width of widget

Hi, I want to widen the width of widget:

I already go here and change:

but it doesn't work! Help plz. Thanks. I'm using phpBB3.

Have you read through this tutorial yet?

Yes, I read it already. But when I set the Width column 3 (right) to 300 pixel it didn't work, it still 180 pixel. Then I go to css and change this

Code:

*+ html #content-container div#main {
   margin-left: 3px;
   }
#content-container div#left {
   width: 0px;
   float: left;
   margin-right: 0px;
   overflow: hidden;
   }
#content-container div#right {
   width: 180px;
   float: right;
   overflow: hidden;
   }

I change 180 to 300 but it goes error. Here's my site http://ddtfc.forummotion.com/index.htm, plz check and help me. Thank you!