   Heightmaps...
  After fighting with models I decided to use .MD2 format... now I was thinking about some terrain. Frustum culling isn't important for me now, because I was thinking, image .RAW representing heightmap can't be big... what is used for big terrains then? or maybe objects on terrain are so small, this way heightmap size isn't big either...   
  .RAW is the smallest format to fit as they have little to no header information, just storing the actual pixel values. They are used for large terrains, although you can even have a 64x64 RAW file loaded into a 256x256 terrain to save space if absolutely necessary ( 1 for every 4 vertices)   