As you have mentioned about PAGE width, for which I have answered above.Also, you have talked about SIDEBAR.So if you want to reduce the WIDTH of sidebar and want to increase CONTENT WIDTH.Here is that you can do.
For Example: If you want to increase the CONTENT WIDTH by 100px, so you will have to reduce the SIDEBAR width by 100px.Here is what to change in CSS.
INCREASE PAGE CONTENT WIDTH:
/* to increase the WIDTH by reducing RIGHT margin by 100px, we get*/
margin: 0 240px 0 0;
REDUCE SIDEBAR WIDTH:
/* to reduce the WIDTH by 100px,we get*/
margin: 0 0 0 -200px;
NOTE :(While ADDING Custom CSS, please use custom CSS plugin (I like my Custom CSS)).
Hope this helps about what you asked.