With Windows Phone 7.8 and Windows Phone 8 it is now possible to customize your Start Screen further with resizable Live Tiles.

Tap and hold a tile for a second or two, and then tap the arrow icon. Depending on the app, the tile can be of two or three different sizes. When you are ready, tap the screen.

You can also rearrange tiles. Tap and hold a tile, drag and drop it to the new location, and tap the screen.