IE7 behaves the same as IE6 in this case. The font-size returned is the same, regardless of the default size set in the browser (...Medium, Larger, Largest)
FF (for which the above font-size test works) has a minimum font-size setting in the browser anyway. This appears to be enabled by default in FF2, but I'm not sure that it is in FF1.5. Op9 also has a minimum font-size setting which is enabled by default.
IE7 has 3 new accessibility settings worth noting...
I would have thought that most IE7 users will be 'zooming' rather than changing the text size, since this is far more accessible (bottom right of main window) and has replaced the Ctrl+MouseWheel shortcut which previously altered the text-size.
A 'font control' which effects just your site. And save the value in a cookie for when the visitor returns. But your font setting should still be relative to whatever default font-size is set in the browser. IE6 doesn't allow style switching in the browser, so you can essentially emulate this with code. But FF and Op do, so I think it is still important to maintain this functionality in those browsers. For the most part it is a nice convenience for your users, and shows you've tried, but if the user uses your control to correct a misconfigured browser then they aren't really solving the problem, but hey. Also, for someone who really needs larger text because of sight issues then they are likely to be already familier with browser shortcuts to change the font-size.