How to change width of post text?

Hi!

I’m trying to change the width of the text in my posts from 750px to 600px. I’m not sure if I should do this by increasing the white space around the text, or by changing the width of the main block and making my website narrower overall. I’d like to know how to do both. My website is http://www.tracingthreads.com

I’ve tried changing the values of the padding and margin here and haven’t seen any difference.

.post, .page {
margin: 0em 0 1.5em;
padding: 0px 5px 5px 6px;

How do I change either of these things? Any help is appreciated!