I have some question concern similarity measure and i need your help (Iam new in statistics)

suppose that we have a matrix M where M(i, j) is the similarity measure between user i and user j .

Question : 1- what similarity measure choose and why?

2- What is the maximum size of the matrix? What are its properties? How would you choose the store?

Thank you very much for your help bests

I have some question concern similarity measure and i need your help (I'am new in statistics)

Suppose that we have a matrix $M$ where $M(i, j)$ is the similarity measure between user $i$ and user $j$ .

Question:

1. What similarity measure to choose and why?
2. What is the maximum size of the matrix? What are its properties? How would you choose the store?

Thank you very much for your help bests