I made a underground house with a tower above ground. Every time I log in the game (single player survival) the tag is gone and the building on the map is gone too. It has lights/campfires and so on. Other areas on the map for mark locations stay there.
For the size, I was bit sad. I find very large forest, good size mountains. The grasslands seem almost same size of the snow areas which is small. At least the North and South directions. I was around the starting area for the game, was in the snow area. I usually travel one full day for exploring. The snow area walked straight north into another forest biome. I thought it would be bigger.