I don't think InfoPath provides a ruler, which I know can be very frustrating, especially when you start getting into nested tables that display differently in the designer than in the runtime.
One thing that you might find useful is to use the built-in magnifier from Windows. To launch this, hit WindowKey+R, type "magnify", then hit enter.
I hope this helps, and if you find out a way to get the ruler in the Designer please tell!
Matt Faus / Microsoft InfoPath MVPQdabra® Software / Streamline data gathering to turn process into knowledge