Hi there! I can definitely see how this would be helpful to some Emma users. I checked in with our Editor Team’s product owner, and looking at the bigger picture, this is not likely something we'll get to in 2014.
If you are using Google Chrome you can add this web app that has a ruler you can move around your screen to help you with sizes, etc. It might have one for firefox as well, I'm not sure. But I use it sometimes. :) Hope that helps!