Ok I didn't see anyone tell you this so the files you were looking for were
K_CityHubs.xml
and
F_CityHubs.xml
In
\Elemental\data\English\Core Improvements
The values you would modify if you wanted to change the tile limit are
<BuildableTiles>50</BuildableTiles>
You can manually increase them to whatever you like.
Before editing them you should copy and paste them into your
My Documents\My Games\Elemental\ folder (folder layout may differ depending on OS), I consider the Tiles folder a good place for them. Edit them there and the changes will automatically be applied when starting a new game
Sharing mods is great, but also sharing the where and how also empowers the user and grows the modding community 