Each icon atlas is partitioned by the individual size of the icon itself. 
Various icon types have their own unique set of atlases and sizes that they use.  

Here are some example sizes:

Promotions		256, 64, 45, 32
Buildings		256, 128, 64, 45
Citizens		256, 128, 64, 45, 32
Civ Symbols		256, 128, 80, 64, 45, 32
Difficulties		128, 64, 32
GameSpeeds		128, 64, 32
Leaders			256, 128, 64
Natural Wonders		256, 128, 80, 64, 45
Policies		256, 64
Resources		256, 80, 64, 45
Technologies		256, 214, 128, 80, 64, 45
Terrain			256, 64
Unit Actions		64, 45
Units			256, 128, 80, 64, 45
Unit Flags		32
World Sizes		128, 64, 32
World Types		128, 64, 32	