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)?