I don't think this is possible (whether server-side or client-side). There may be a method specified in the official DOM (i.e. by detecting the font-size setting of the current style sheet), but even if that is the case, the majority of current browsers would not support it.
Kevin: After some researching, I have to agree with you. Font styles / sizes are user specified. Perhaps with the browser capabilities component? Probably not, but it's the closest thing I can come up with (in ASP, anyway).
When you say "detect text size" what exactly do you mean. If I'm thinking of the correct thing, the best I could think of is define layers with different text sizes with CSS that won't change depending on the user's "text size." Then compare the widths of those layers with the width of a layer with text w/o any size defined (it'll use browser default).
I'm not sure i really need to do this anymore, guys. I was planning to dynamically position my DHTML menus according to the text size the user has their browser set to. So, if the text size of the navbar was set differently on a number of clients, the menus would automatically readjust their position. I wanted to do it this way rather than traditional event-handling because I wanted as much and as easily as possible to create a completely cross-browser solution, since all browsers seem to have their own methods of event-detection.
However, I've since had to compromise and consequently returned to using event detection, instead. At least, in IE4+ i have. I've had trouble getting this to work in Nav and NS6, despite using their respective browser-object models. Anyway, I've managed to address this by passing a 'this' parameter when the user clicks the appropriate hyperlink on the navbar. This just find the x and y positions of the hyperlink and then draws the correct menu at the specified position. Not exactly ideal, but it works.