You can set the background size and scaling attributes in the Advanced tab of background properties that you can access by clicking on the Settings Gear icon on the Background Actor. Then you can experiment with the 3 settings - Normal, Stretch and Tile, and see how they change the runtime behavior of backgrounds.
In addition, the 2 checkboxes below that enable the tilemap and background scrolling are very useful when you use the Level Editor and build a game that needs to scroll beyond the screen. They are usually enabled by default if you use the level editor and the platformer library of blocks. Blocks such as "set camera to me" are very useful to build games that scroll. You can look at the Platformer Tutorial for ideas of scrolling.