PLEASE HELP

Hello every one!

I want to widen and center the post box of my blog. (I mean the white part of the page where the post is in)See it here: http://contradiccionesdelabiblia.com/%C2%BFcontesta-dios-las-oraciones/

I want the post box to be like the pages are in my blog. See it
http://contradiccionesdelabiblia.com/sample-page/

How can I achieve that!
Any advice will be much appreciate!

THANKS in advance