When matchmaking for clan wars the first thing that happens is all clans are split into groups based on size. Smaller clans have narrower bands than larger ones, but you should always be in a banding that is competitive.
The bands are then split into divisions according to the overall performance of those clans. Historically this has covered wins recorded in a certain period, overall win/loss percentage and whether your clan finished top in a previous clan war.
It's worth noting here that the actual method used to decide divisions, and the sizing bands for clan size change slightly from clan war to clan war as we refine the system in use. This is the primary reason we haven't explained exactly how the system works, because ultra-competitive clans will target having the exact right number of people in time for the roster lock in order to fit a certain grouping only to find the bands have changed a little.
We know you'd sooner see the exact science behind matchmaking, but until it settles down into one system we won't reveal the exact method as any information we give will be inaccurate very quickly.