Operation Flashpoint OFP terrains are called islands. Default BIS islands are all 50meter cell size and 256 x 256 cells per island making them 12.8km x 12.8km in total size. Islands are made out of WRP files, to edit WRP files get WrpTool which is hands down the best island editor available.

OFP Resistance texture descriptions explains what type of terrain each texture represents.