Good afternoon!
[tex]1L = 1 cm^3[/tex]
So, to discover the 'area', you have to fix the thickness of the 'spilt milk'
Another idea is: If you have a room with 3m x 3m = 300cm x 300cm, you can calc the area... and discover the thickness, right?
Like this:
[tex]V=abc\\\\
1=300\cdot 300\cdot h\\\\
h=\dfrac{1}{90\,000}\approx 1,1\times 10^{-5}cm=1,1\times 10^{-4}=1,1\times 10^{-1}\mu m[/tex]
I hope I have helped!