Changing the Artboard Name
At the top of the Artboard, you can edit its name. It's located just above your image, below the top Navigation options.
Change Artboard Background Color
In that same area, you'll see the background color tile. Click on it to open the color picking function:
Changing the Artboard Size
You'll see the current aspect ratio you're working with to the upper right of your Artboard.
Click on that to open the selection menu for your size options.
*Note: You can't launch an experiment if it contains Artboards with sizes that don't belong to the platform you're launching on. For example, if you choose Google Ads image sizes but are launching to Facebook, the Experiment will not launch.