You don't. Mapconv will do it automatically if the texture has repeated area. Basically, if you have a big red rectangle that is 64x128 in the corner of your map, Mapconv will automatically break that into two 64x64 chunks and only save one of them.
So it occurs pretty much automatically, you make your texture repeat in non-obvious ways here and there, and the file size will decrease.
The tradoff, of course, is that you have to do it well or it'll look obvious and ugly on your map.
