You'll get a neater splice if you use the same size wire as that you're replacing. Since the wire in the stator and up to the point you splice in limits the current flow anyway it would be better to continue with that size. Also that way any heat that may be generated will be dissapated along the whole length of the wire not concentrated in one spot.
I used either 12 or 14 for my splice, I also used Heat shrink Butt connectors, seemed to work good and will help make sure that the water stays out. I also would suggest replacing the dog bone fuse, with a 30amp blade. I used a holder with the rubber cover on it.