Exactly 16384 units in each dimension. This makes it 8 times the volume of a HL1 map. But mods can use difirent scalling. I.e. you can make 1unit to be equal 10centimeters, wich will make the side of a map about a mile long and that's a lot. Yet it's nothing, since some mods use scalling wich allows a map to be 50 square kilometers... (imagine the whole HL1 single player in one HL2 map)