The tilemap docs are indeed a bit lacking due to it being in beta. It will improve massively in the next version (2.1).
1. You can't resize tilemaps from code, this is intentional, but you can safely create a HUGE one to start with, the overhead for an empty tilemap is very very low.
2. You can change tiles on a tilemap after you get a reference to it. eg. tilemap.Layers[0].SetTile( x, y, 1 ); tilemap.Layers[0].GetTile(x, y ); returns a tileId. These functions are likely to change in 2.1, though, which is why they aren't documented! Call tilemap.Build(); after you change what you need to.
3. There are also some documented functions in tk2dTileMap.cs which you can find by browsing the file (look for <summary>) - as the APIs aren't final, hence the reason it not being in the docs.
Hope that helps, and glad you're loving 2D Toolkit