In the mtext dialog box, select the fraction and right click. If it is not already stacked, select the stack option. Then, when it is stacked, you can select it and right click and select properties. Then there is a button for "Autostack"
However, even after all this, I'm not so sure you can achieve what you are tying to do, because even the dimension just uses mtext. So I bieleve it is all or nothing. I could be wrong though!
Chris