Will clans of different sizes be matched for war?

That would be unfair. Both clans will always have the same number of war bases in a clan war. Clan wars matchmaking will only match clans of similar sizes, and a few members (between 1-4 per clan) may be put into spectator mode when the clans are matched to even out team sizes. The members in spectator mode will be those with the lowest trophy count in their clan (or those who are ineligible for war).

10 vs 10 members is the smallest possible team size for a clan war. Other possible team sizes are: 15 vs 15, 20 vs 20, 25 vs 25, 30 vs 30, 35 vs 35, 40 vs 40, 45 vs 45 and 50 vs 50. Clan wars matchmaking will always match your clan into the largest possible clan war.