In HTML at least, changing font sizes in the browser has more
to do with the styles than with RH's 508 compliance. Current
standards are to style text using percentages rather than fixed
point or pixel values for sizes. So this is up to the help author
rather than being something RH will manage automatically.
Apparently, browsers contain settings that allow the viewer to
override text and color settings (see
this W3C page). But
its nice to not force them to do that.
When it comes to resizing text on skins or in the HTML Help
interface, someone else would have to comment on that, as well as
the rest of your question.
--Ben