Here's how you do it: 

For iOS users, if you are using the Freestyle Mode, please see option Layout in the bottom left corner. Tap it and see option Size on the left hand. 

By tapping Size, you will see three collage size options 1:1, 2:3 and full screen. Please choose the one that best suits your creation by tapping the option directly. 

If you're using Grids Mode, after selecting photos, you will be led to a selection of grid layouts. See option Size on the left hand and you'll see all the collage size options we offer!

As for Android users, when using the Freestyle Mode, please see the grid layout button in the bottom left corner. Tap it to see different grid options. On the left hand you'll see the option to change collage size. 

Please see the following instruction collages for reference. :-)

[iOS]

 [Android]