create a variable to count customer by size

Hi

I am new in BO, and trying to create a new variable to calculate how many customers sell the two size of the brand. My data is

customer   size      sales

x               750 ml     1

x               1.75L       1

y               750 ml     1

y               1.75ml     1

z               750ml      1

so my result must be 2, because the customer z only sold one size.

How can I make it?

thanks