How to change page margin size / Twenty Eleven theme

more direct link to one of the pages, which is where the issue is:
https://jessethom.com/bio

All we’re going to do is look at Firebug (or equivalent) and find the CSS that’s defining your width [screenshot].

You can post CSS queries on forums like https://csscreator.com/forum

thanks Andrew. i notice the screenshot you posted is of my front page, as opposed to the other pages (where i’m having the issue). do you know if the div#page tag applicable in that case as well?

also, i’m new to firebug and though that may be all you can help with, it’s a big help because i am likely missing something.

Check on the HTML mark-up errors on that page:

https://validator.w3.org/check?uri=http%3A%2F%2Fjessethom.com%2Fbio%2F&charset=%28detect+automatically%29&doctype=Inline&group=0

do you know if the div#page tag applicable in that case as well?

Yes, and you can find out for yourself through Firebug.

ok, thanks Andrew. I’m grateful for your kindness with offering some help.

@jthom, the problem is almost certainly due to the mark-up errors on that page – those sorts of errors interfere with how the CSS is working.