Live Tiles are definitely the most iconic feature of the Start Screen. Windows 8 only offers two sizes: a square and a double-sized rectangle. Windows 8.1 adds two more sizes. One is smaller than the current square, while the other is larger than the current rectangle. The Tiles are now referred to as Small, Medium, Wide, and Large.
Small Tiles are roughly equivalent in size to regular icons, making them very useful if you want a denser, more mouse-friendly Start Screen. Unfortunately, these Tiles aren’t Live, so they won’t update with information like the other sizes. However, this makes them great for apps that don't need to be updated, and perfect for Desktop apps that lack Modern UI tiles at all.
On the other end of the spectrum, Large Tiles can display twice the information as Windows 8’s big, rectangular Live Tile (now referred to as “Wide”). These are handy for reading the contents of your latest email or displaying an upcoming Calendar meeting directly on the Start Screen. And for you desktop-only users, should you need to escape the Modern UI in a panic, the Desktop Tile can now be made to dominate the Start Screen.