what size megafuse?

On the charging circuit, the fuse should be 130% above rated alternator output. i.e. If your using a 50 amp alternator, you'll want a 65 amp fuse. Multiply output amperage by 1.3 to determine charging circuit size. Or, use a fusible link wire four gauge sizes smaller than a properly sized charging wire (by alternator ampacity) on the charging circuit and use an AGC fuse block on the battery positive for the small circuits.