It will depend entirely on the esc and motor draw to what size fuse you will need.
Generally you just fuse the positive motor cable, (each motor if you have 2 or more) although its been known to fuse both the motor and the battery for that extra protection, but i doubt you will need to go that far, as only running 6v.
As an example if your running a 20A esc with a 555-600 size motor with a 40-45mm prop you should fuse the motors positive with a 15A fuse.
What esc, motor(s) are you going to run? if you can provide this information im sure we can provide the correct information.