Similarity measure

matrix

#1

Hello,
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