The fact that you are asking this question makes me think you have never started a war.
When you start a war, the first thing you do is select the 5,10,15,20,25,30,40, or all 50 clan members you want in the war. That is what this quote:
The matchmaking system in Clan Wars puts your clan against another clan of similar strength both offensively and defensively, and of the same size, rounded down to multiples of five (or ten if member amount is 30+) eligible members.
is referring to. Those are the "rounded down" members they are talking about.