Suggestion for different blog post layouts

Hi there,
I am finishing off a site that another developer worked on.
They added the CSSe below to style blog posts – it was 700px wide, but I changed it to 800px (and reduced the padding).
But I also want to provide the client with a 2 column layout, but of course 800px is too narrow.
But there are existing posts that work fine at the 700/800 layout (a single column)

What would you recommend to do to provide a flexible way to have both options for the client going forward for new posts?

TIA, Dave

.single-post .site-inner,
.single-post .content-area,
.single-post article,
.single-post .entry-content,
.single-post .gb-container,
.single-post .gb-grid,
.single-post .gb-inner-container,
.single-post .comments-area,
.single-post #comments,
.single-post .comment-respond {
    background-color: #f7f4ee !important;
    padding-top: 5px !important;
    padding-bottom: 15px !important;
    padding-left: 0px !important;
    padding-right: 0px !important;
    margin: 0 auto !important;
    max-width: 800px !important;
    box-sizing: border-box !important;
}