If there is unused space on the canvas, the background color by default becomes black.
Most Wavve users will create an image (using a site such as http://Canva.com) that spans the entire canvas space.
Below are the recommended dimensions for each design size:
Square: 1080 x 1080
Vertical: 720 x 1280
Horizontal: 1280 x 720
For example, if you were working with the square design size and wanted a plain white background, you could create a 1080 x 1080 image that includes your logo and has a white background.