I'm new to electric flight, and am not sure what battery to get for the cobalt 400 motor. What size/voltage would work best, how do you know which size/voltage to use?

Do a search on the motor and try to get specs for it. I'm assuming its being used in a plane so determine what kind of performance you want. This is a trade off because larger capacity battery = longer run time but weighs more. Your motor will give more RPM's with more cells and will also burn out early if you go to high on voltage or load. Your prop selection comes into play here. Basically there are many factors to concider so your best bet is to utilize these forums, do a search on your plane/application, and see what others reccomend for best results. Good luck.