I agree iKay. Currently you do not get any benefit from resizing the window. If the contents do not scale to fit the window then there is no reason for allowing the window to be resized. On the other hand, if the window can be resized then the preview should also resize, as you say.
However, I have to ask: Why do you need more accuracy? Is there a specific scenario you are working in where accuracy is important? (For absolute accuracy you can already give specific numerical values.) I'm curious as normally you would click the preview page somewhere above what you want to point to so absolute positioning isn't generally necessary. I am not saying that accuracy is not needed, I am just wondering why it should be. Maybe there could be a better way to do what you want.