Neither Opera 7 nor MSIE 5 fire the resize event on text resize. Even though both browsers reflow the page. I think this is a bug in all three browsers, but I thought it worth mentioning.
The resize event means the window actually got resized. In this case, it did not. Firing the resize event in this case would actually break pages that expect the resize event to fire when a resize actually happened... There are many reasons a reflow could occur that have nothing to do with the window being resized; font size changes are just one way it could happen. It sounds like you want to detect all such, which would require a rather more comprehensive solution than just ad-hoc firing of resize events.
It might be worth pointing out that Netscape 4 *does* call the resize event when the font size is changed.
That looks to me like a hack netscape 4 had to put in to make itself reflow the document (it catches the resize event itself and starts a reflow, then notices there is a new font size set in preferences). Any sort of modern browser has style-dependent ways of triggering reflow in general, so doesn't need that hack...
I'm implementing a "documet overview" feature where a script displays a small copy of the large main document (think spreadsheet or map) fixed in the corner of the screen. On top of this a partially-transparent coloured block indicates which part of the document is currently visible. This updates on scroll and resize events, and ought to update when the text size is changed, if that were possible. I'd support firing either scroll or resize events in this case, or inventing some new mechanism.
See also bug 303405, "Dispatch an event when the text zoom level changes".