I thought I could easily figure this out but am finally losing my mind... so Help, please!

The page width for the site I am building is 1600px, which I have set in the ATO / Layout. The posts, on both the single posts and the homepage for posts needs a width of 1000px. When I try using using left/right margins, e.g.

div.post {margin-right: 300px;}

but the right sidebar (used only on posts, not used on pages) stays far to the right. I need to bring both left and right sides in 300px each.

I've tried setting the layout width to 1000px and changing each page's width in CSS to 1600px but having been unsuccessful using variations of:

.page-id-51 #content {
margin-left: -300px;
}

or

body.page-id-11 .container {
max-width: 1600px;

but have had no success. I only have 5 pages so it's not a problem to style them individually, if I could only get it to work.

What am I missing that should be obvious? Thank you or your help.

Here is an easy way to solve this
1) get rid of the CSS you have entered
2) at ATO->Style & configure SIDEBARS->RIGHT INNER sidebar WIDTH set it to '300'
3) at ATO->Style & configure SIDEBARS->RIGHT sidebar: Display on: UNCHECK
Front Page
"Page" pages
"Not Found" (your choice)
Attachments (your choice)
4) at ATO->Style & configure SIDEBARS->RIGHT INNER sidebar: Display on: CHECK ONLY
Front Page
"Page" pages
"Not Found" (your choice)
Attachments (your choice)
5) go to Appearances->Widgets->Right Inner Sidebar and add a 'TEXT widget
6) in the contents of the TEXT widget enter ' ' (this is a blank character)

You are all done.

So now you will display the Right Sidebar on the pages with posts (blog, author, tag etc) but not the RIght Inner side bar. On teh PAGE pages you will display the Right Inner sidebar and not the Right sidebar. You have to add a text widget to teh Right Inner sidebar so you don't get the three default widgets

Wow! Thank you so much for the rapid response and an easy peasy solution. So much better (and successful) than the direction I was taking!

Glad to be of help. I'll mark this one solved.