Config.cpp for terrains, the name of your terrain. This data is from ArmA 2 version 1.62 user interface, other version might vary (hopefully not though).
Terrain name can be only so long until it gets cut off.
Main menu → mission editor → terrain:
“Tech Demo Terrain 6, 409”
Mission editor → load:
“Tech Demo Terrain 6, 4096 grid 10m cell, 3.8 Million Ob”
So main menu name length is 24 characters, mission editor length is 55 characters.
You can have of course really long name there, but nobody is able to see it from these two menus, unless they have some custom made addon to change user interface. You can pull the terrain class name from missions of course, but that's another matter.
Most cases though the terrain names are fairly short and there is no problems regarding the length.