Changing content/ post-body size in Simplemarket default page template

I'm trying to change the size of the post-body area (content) in the default page template in Simplemarket (with no luck so far).

Basically, I would like it to look more like the blog page, with a width of 750px for post-body:

This is how it currently looks: with the text in the body moving down the page and starting below the sidebar.

I've tried changing the code where I thought I should, but haven't found either the right place, or the right code. The post body is currently set at 1042px for the default template, as you can see in the screenshot, I know this needs to be set to 750px as in the blog template, just a bit unsure as to how to make that happen.

Many thanks in advance!