Using information from Duelist101, I decided to make this thread. I will show you lists of each world by length using different categories, and this will not include side quests.
First Arc Worlds by Length
Wizard City: 39 quests total
Marleybone: 50 quests total
Krokotopia: 65 quests total
Mooshu: 91 quests total
Dragonspyre: 106 quests total
The first arc has a total of 351 quests and an average of ~70 quests per world.
Second Arc Worlds by Length
Celestia: 95 quests total
Zafaria: 148 quests total
Avalon: 161 quests total
Azteca: 197 quests total
Khrysalis: 277 quests total
The second arc has a total of 878 quests and an average of ~176 quests per world.
Third Arc Worlds by Length
Empyrea, Part One: 79 quests total
Polaris: 99 quests total
Mirage: 113 quests total
The third arc has a total of 291 quests and an average of ~97 quests per world.
Side Worlds by Length
Wysteria: 38 quests total
Wintertusk: 51 quests total
Grizzleheim: 69 quests total
The side worlds have a total of 158 quests and an average of ~53 quests per world.
Now, this all seems pretty boring. But it gets really interesting when you throw out the categorization and plainly list each world.