Custom WordPress Site

You can change the width on virtually all themes / pages — depending on the coding, sometimes it’s very simple in the CSS styles, other times you might have to modify the php file.

If you want only the one page to have a custom width you can either isolate it with css using the body_class() output: .page-id-(your page id) {width:1000px;} or you could create a custom template that you choose when you create the page. See:
https://codex.www.ads-software.com/Template_Hierarchy#The_Template_Hierarchy_In_Detail

I think I want to use the body_class()

Would I add this as a new definition in the stylesheet or add it into the html of the page or change the php file?! sorry i’m a css amateur.
Thanks for your help thus far.

It needs to be added to your theme’s header.php template file. See https://codex.www.ads-software.com/Function_Reference/body_class

How exactly would I add it into the header.php

I’m really dumb

Please try reading the page at the link I posted above – especially the Examples section (big hint).