So I finally found the solution for this problem. What I did was, entered in apperance/costumize and changed the layout to site width 992px and main content width 75%.
Then, I open each one of the pages edit I wanted to remain with full width and insert in the text area this:
<style type=”text/css”>
#primary {
width:100%;
}
</style>
I could do it in the other way, but since the pages I want the sidebar are categories it wasnt possible to insert that code.
It worked like I wanted, finally. I just have now a little problem that is the gallery I have in one of the pages still acts like the main content width is still 75% and now 100%.
If anyone have a suggestion how to resolve this tiny problem please tell me. In the mean while I will try to fine a solution.