I'm guessing you haven't drawn any tiles to that tilemap object. ?? Because you'll see them as you draw them in the editor. If you look at the Tilemaps sample project, in the Beginner Examples, you'll see that the Tilemap object is drawn to the layout, like you've done, and it also has a tileset, displayed in the editor by the Tilemap Bar. Click the pencil icon in Tilemap Bar, select a tile from the set and draw it on the layout, in the Tilemap object. (Maybe not the clearest of naming conventions there, but still pretty simple. :-))
And you ask about resizing tilesets in the editor, and I don't think that can be done there. Better to do it in a bitmap tool like Gimp that can do a good job of scaling.