Scaling objects relative to one another

Is there any way to redefine objects as 100% once you have made them proportionally as desired?

i.e take the "plane" default object and shape to a rectangle x:y ratio and then define that as x=100% and y=100% so that you could size other distorted objects to match its size and proportions?

Also anyway to scale a decal to the size of the surface its mapped too. Create a panel, size it to ratio 2:1 and then import and place a perfect 2:1 ratio graphic to its surface.  All measurements seem to be related to the base model and not the scene, distorting decals and making objects match each other for size a random spate of guesswork.

Its an improvement on Felix, but lacks the simple repeated visualisation options of something like Boxshot 3 or 4D where you can, once you have a model, swap in similarly sized decal artwork and quickly re render.