Frank....I would place the fuse in the positive lead. You can size the fuse for either the ESC or motor which ever is the lesser of the two and I would use a slowblow fuse.
Now personally I don't use fuses as I've had to go in after my model because the fuse went. I know some will tell me I'm foolish or even stupid, but so far I've never burned out any ESC 's or motors. If I can move I just park the transmitter stick in the neutral postion till I figure out what is wrong.
Jim Wheeler....."Life is not measured by the number of breaths we take, but by the moments that take our breath away."