Layout Option

Hello! I tried for awhile having my web page with no sidebar and full width. I liked the size of the images but felt that the text stretched too much. So I changed the settings to "No sidebar". The text is much more centered now, but the pictures are a little smaller than I would like to. The pictures are currently getting scaled down to 800pix, I would like the pictures in the feed/blog to be 900 or perhaps 1000pix. Do I have to change the width of the mainpage and how do I do that? Thanks in advance! :)