Go to Settings tab on the left side of the application.
Select Custom Size from the Size list in the Layout section.
Set width and height values.
Set a resolution (pixels/inch) for your collage. The recommended value of resolution for high quality photo prints is 300 pixels/inch and for photos to be posted on the internet is 72 pixels/inch.
Set a name for your custom size.
The default resolution is set to 300 pixels/inch.