Hi there, I've scoured the web and it seems it is possible to declare various widths based on the template/page, for example if the page was using the one-sidebar-right layout you could specify a width for it. However it seems it would need a complex hook which would mean more code, which was something you were trying to avoid. However, just in case I've attached link below in case you want to take a look at it. To be honest I'm not sure how that would be implemented in the framework, or even if it would work 100%.
As usually the content width only gets declared once, I would think the safest thing to do would be to declare the width as the figure for the sidebar layout, that should then work on both pages without formatting issues, although on the full width page it wouldn't stretch the entire page width, if centered it might not notice too much, depends on the size of the sidebar.
If you can set the width to the sidebar layout version so we can see how that looks on a full width page, we might be able to see if css could help in adjusting the width a little more to make it sit on the full width page better.