How can I resize one layer to fit another?

[Ofnuts]

While Blighty's answer is technically accurate, for your application I suggest using my ofn-layer-aligner script: you use the Path tool to mark two points on a "reference" layer and two points on the "adjusted" layer (typically in your case, the center of your pupils),. Then you call the script that scales, rotates, and translates the "adjusted" layer so that the points you marked on it exactly match the points you marked on the reference.