How can I resize one layer to fit another?

[dblj]

I am trying to do this project where I take a photo of my face every day, and I want to stabilize my photos using GIMP. In my first picture, my face is more zoomed out than in the photo I want to edit, so I want to shrink the photo I am editing to fit that. How can I change the size of a layer? 

Similar to this instance of this video: https://youtu.be/FGXGHYonfGM?t=178

[Blighty]

ideo is nearly 5 minutes, didn't watch it.

To scale a layer:
Use the Scale Tool  
In the Tool Options check Keep Aspect
Click on the layer to be scaled and drag a corner.

[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.