LBurger

Actually this is what I needed to add to the child CSS to change the content width of the specific page:

.page-id-231 .entry-content {
max-width: 800px;
}

Yes, that’s what I want(ed).
Downloaded and installed Firebug. It helped me better than I had hoped from previous experiences. I checked the page and found the area I wanted the width to change. It took me a couple of tries to address the right parameter in the right way, but I managed to change it.

Thanks very much for your advice. Learned something again.

Andrew,

Yes I have seen those. Didn’t use Firebug, but loaded the page source into my off-line editor and tried several options based on the page structure and the hints I had seen on the forum. No luck.
I then thought that because of theme-specific tags and classes the options might work for other themes but not for Twenty Fourteen.

I will install and try Firebug, but I am not really optimistic that I my findings will be very much different.

Peter

Had the same problem.
The first line did it for me. Thanks!