Post width suddenly reduced, no idea why

Have you checked for mark-up errors – they can mess the layout and/or how the CSS is working –

https://validator.w3.org/

I tried to test that page, but got a “not found.”

Sorry it was set to draft.

try this

https://www.macsmagazine.com/2013/02/test-2/

it has 50 errors but none i can see relate to the post content.

Not all of the errors are necessarily a problem, but you’ve got quite a few: https://validator.w3.org/check?uri=http%3A%2F%2Fwww.macsmagazine.com%2F2013%2F02%2Ftest-2%2F&charset=%28detect+automatically%29&doctype=Inline&group=0

The ones having to do with tags or elements out of place are a likely cause.

Do you have any idea why would effect only new posts and not old posts?

Possibly you added some HTML in your new post without realising.

Or removed a tag inadvertently.

Thanks what i first thought but its happening on every post that I create. even if i just type “test”.

Thanks for your help guys

If it doesn’t look like the HTML or errors, try using Firebug to see what’s going on with the CSS affecting that element of page section.