Non-linear regime of matter power spectrum

Question 1

I was trying to understand why cosmologist calculate the matter fluctuations in spheres of $8 h^{-1} Mpc$. I look for information and I find that in this scale the fluctuations become non-linear, but I can’t understand how they conclude that. Why not calculate the matter fluctuations in spheres of $0.8h^{-1} Mpc$?

Question 2

Can be explained in a simple way (without calculations) how cosmologist calculate the matter power spectrum? Is the matter power spectrum product of observations or is a theoretical derivation?