With the release of the new CreativeStudio, users now have the ability to create their own custom design area. This will allow you to create a document that fits within the exact specifications you need for your particular creation when a template is not available.
To set a custom design area, please see below:
1) Click the 'More' item on the bottom of the left column of icons, as pictured below:
2) This will reveal the Custom Print Area option:
3) Clicking this icon will reveal the dimensions panel immediately to the right:
4) Simply enter your dimensions, then click 'Create'.