Help with bigger pics in blog posts, leaving the front page alone?

Hi,

I’ve found some suggestions but I haven’t been able to do what some people have done (for lack of programming skill).

On the thread below, I learned the CSS to make my pictures big:

https://www.ads-software.com/support/topic/full-width-post-for-oxygen-theme-1?replies=10

However, I only want my pics big in my blog posts.

I removed the visibility of widgets in the third column only on “Pages that are Posts,” via Jetpack.

Should I alter the post.php or singular.php files, make copies of them and put them in my Child theme folder?

The only thing I want is to make the pics in my blog bigger and no third column. I want to leave the front page alone.

My website is interior voyeur dot com.