It depends on your connection, but in most cases the optimal settings for streaming on Bandcamp Live are:
Audio quality / bitrate: 192kbps
Video quality / bitrate: 720p at 2500kbps
FPS (Frames Per Second): 30fps
Aspect Ratio / Visual Ratio: 16:9
You can preview your stream before you go live to ensure that everything looks as it should and your connection is stable. Here's more information on testing your connection.
With a powerful streaming computer and strong internet, you may be able to stream at 1080p, but be sure to test your stream thoroughly before going live.