Your posting isn't overly clear, so I'm guessing a bit.
I'm, assuming that "they" (whoever they are) are using webmail and that they have downloaded (not uploaded) and installed Chrome (so what was being used before?).
That all being the case, changing the font size in Chrome's settings will not affect the font size in an email created (using webmail) via Chrome. It will only affect the font sizes of what is displayed on the screen using Chrome. Instead "they" will need to select the font size within the webmail webpage of the webmail provider.