Alliances are matched in World War based on the strength of their members. Medals from Multiplayer are not a factor for World War matchmaking. 
How is strength calculated in a World War?
Strength is based on each member’s attacking power (Troops and Barracks upgrades, War Tactics, Generals, etc.) and defensive strength (defense buildings, walls, traps, generals, etc.). Base layout does not affect this calculation. The only way a member’s strength can be increased is if they upgrade their troops, defenses, war tactics, or other contributing offensive and defensive capabilities.
How many players can participate in a World War?
The smallest war allowed is 10 vs.10 players. Other war sizes include: 10 vs. 10, 15 vs. 15, 20 vs. 20, 30 vs. 30, and 50 vs. 50. Any Alliance members that are not chosen for a war will be observers until that war is over.
Why is it taking a long time to find a match?
Opponents are chosen to make wars as fun and challenging as possible. Unlike Multiplayer, you may not find an opponent right away. If you no longer wish to wait, you can cancel the search by tapping the “cancel” button on the World War screen (Alliance Leaders and Co-Leaders only). If you haven’t found a match in 24 hours please contact us via the in-game support system: Settings (gears icon)> Customer Service.