In Windows 8.1, the tile sizes on Start Screen have been increased from 3 to 4 and the users are also given the option to change the size of the tiles.
Before Resizing and/or Moving tiles, you need to customize the Start Screen. To customize your Start Screen, follow the instructions:
Right Click, anywhere, on the Start Screen or swipe from bottom. A Customize button, similar to the one displayed in Figure, given below, will popup

Here, press Customize button.
Just see, your customization has been enabled.
Now, lets resize the tiles.
Resizing Tiles Tiles of the Start Screen before Resizing:

Now, to customize the tile size of one or more apps of Windows 8.1, follow the instruction, given below:
First of all, enable the Customization.
Now, select the tiles you want to resize.
After selection, press the Resize button in the bottom app bar.
Here, you will be given 4 different options for tile sizes, such as:
♦ Small - Medium - Wide - Large
Select your desired size.
Selected apps would be resized according to the selected option and their current size.
Tiles of the Start Screen After Resizingsource:windows8core