It seems an opinion whether or not, as web-developers, we should design websites taking into account the user increasing font-size (e.g. on Chrome: Chrome>Settings>Customise fonts). (Although it was last suggested to me that most people do not).

However, going on the basis that we do:

I presume people test by increasing the font-size of their pages at different viewport dimension sizes. Are there any guidelines as to what font-sizes people blow the font up to (for any given viewport dimension)?

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Modern best practice for this is to use the "rem" unit for layout, media queries, etc. where you can.

You should expect that some users will zoom in on your page (Ctrl +). Zooming is similar to increasing the base font size in settings (and I would argue is likely more common, though I don't have any data to prove that). Using a unit like "rem" makes its so that in either scenario your layouts can adjust.

As far as guidelines, WCAG 1.4.4 asks that your site can adjust to at least 200% zoom without breaking to help those that need larger fonts due to low vision. Seems reasonable to extend that to a 200% increase in font size as well.

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