Hello there Alistair,
I may have this wrong as it sounds a bit simplistic.
I made a section.
Inside a div background red (div1)
Inside that another div background blue (div2)
They were the same size.
I then made div1 width 500px
div 2 narrowed to that size
I made div 1 height 500px
div2 didnt change.
So I believe you kind of answered your own question,
Div2 (the one inside) make the height and width 100%
Then what ever the outer (div1 red) does the inside will expand to that size.
If that helps tick the heart and solution.
If I wrong let me know.