MapSizeDefs.xml from the newest expansion pack's Game folder. (EXP4_Retribution if you have that, EXP2_Crusade if you only have this etc).
Copy it to ...Documents\My Games\GC3Crusade\Mods\Example Mod\Game (Example Mod name can be changed to what ever you like)
Open it up in a text editor, Notepad++ preferably for proper formating and less chance of typos. Locate the map size you want to change, say Insane, look for <BaseAdministrationPoints>6</BaseAdministrationPoints> change 6 to what ever you want.
Save, boot GC3, enable mods in the options and restart GC3, play.
If you have Intrigue you may also want to change colony limits which are located in the GovernmentDefs.xml in the EXP3_Intrigue\Game folder. Copy, Paste, Edit etc.
To remove the limit completely copy ShipComponentDefs.xml and remove the Administration requirement table from the Survey, Constructor and Colony modules.