We will need a little more information to give you a solid recommendation like voltage, actual continuous power consumption, and the real peak power you see when riding......which is usually when you go full throttle from a stop on a really steep hill in thick grass. But for reference I normally run an 80-100 amp maxi fuse on similar setups with no problems.
There is some good discussion here and elsewhere on the forum on this topic: http://endless-sphere.com/forums/viewtopic.php?f=7&t=57227
Good luck!