I just tried this with no luck...the labels scale stayed the same.
Maybe I did not do it correctly, these are the steps I took.
Changed the plot scale from "1 to 1" to "1 to 2".
Scaled up the tittle block and the viewport to fit paperspace.
Finally, I set the vport scale to half of what it was to fit the viewport.
Windows 8.1 - 64bit
Intel Core i7 Turbo @ 2.30 GHz
16GB Ram, Civil 3d 2014 64bit .