You can select default sizes from the combo list on the left or directly from templates you have created.
If it's not there, you can enter the desired values in the dedicated fields. Be cautious: if you want to change the document size, remember to uncheck "keep ratio".
You can also resize using the canvas preview to create a document more intuitively.
However, as Black Ink manages very large documents, ensure you have enough disk space by checking the file size of the image at the bottom.
Finally, you can set a background color by adjusting it in this way.
Once everything is set, click OK.