First off to be kind to your readers who happen to be on slower internet connections I recommend going to settings > reading and setting your site to show 5 posts per page. Many are still on slower connections under 1.5Mbps and they will appreciate it on image heavy sites such as yours.
Oulipo is a little strange as all the spacing and widths are done in EM units, which are not a set size, but based on the font sizes used in the theme. This makes it a little difficult to determine what the resulting maximum image size will be. Changing the widths in the three following by the exact same amount will widen the main area, but what you have to be careful with is that over 40% of users on the internet are still on 1024 x 769 monitor resolutions and if you make the overall width of your site 1200px wide, that means people are going to have to scroll horizontally to be able to see the whole page. That is one of the top 10 things people dislike on the web, horizontal scrolling.
What you would do is increase all three widths below by the same amount (example 65, 55, 35). If you did this, the width of the post area would go up by around 16%, so in the "maximum image and video width" field at the bottom of the CSS text area, you would take the standard number and increase it by 16% and enter that number. Again though, you always want to consider your visitors when widening a theme, and you don't want to cause a bunch of them to have to scroll horizontally to see your stuff.