Well, there is text.textwidth and .textheight. But it only works in the next frame, after the text is drawn, but you can probably work around that.
For example you can create a dummy string on screen with the same dimensions as your textbox, set its opacity to 0, set the text to all the text that comes before your word of choice, then check for textwidth and textheight. Your words position then is originaltext.x+dummytext.textwidth etc..