Guidance on max width

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IATI websites are quite variable in how wide their content is, from d-portal at 960px to iatistandard.org at 1200px.

As far as I know, we have very little data about our users screen sizes, as this isn't something that Plausible collects.

I heard from @notshi that making the d-portal tables smaller wasn't an option as they are well-crafted to be their specific size. To add the proposed side navigation to d-portal (at the proposed 25% ish of page width) would take d-portal to 1280px. Is that too big? Can we rely on responsive design to handle slightly smaller screens cleanly?

Conversely, some applications (especially any with large tables or graphs) might be better if users can expand them to their full extent - I don't have a 5K monitor for nothing!

Thoughts, reckons, best practice etc all welcome