I know this is a pretty old topic but I can't let this assertion pass.
First of all @Raskolnikov idea is brilliant, and to implement this wouldn't actually use no more resources that what you are using currently. Well for real it will, but that CPU usage difference can't be even measured on nowadays computers.
We are talking about adding a couple of simple math operations here, and the result will be added to the function that shows current size, so there is no need of extra screen redrawing. Btw values can be updated only when size updates.
What you said about zoom and accuracy isn't true also. Say that is like saying that the shown measure's accuracy will depend on current zoom. What would depend on current zoom is the resize accuracy, and this will affect both, real size and percentage size.
I hope to see this feature implemented soon. I'm really tired of keep doing maths outside the application to know what percentage I'm using to enlarge/reduce things.
Best,
Gustavo