The STS2 power line is only used to run the unit’s circuitry, it is not used to power the turn signal lights. STS2 requires very little power (I read somewhere that it only uses 250 mW). So, a 0.5 A fuse should be more than enough. The package doesn’t come with a fuse at all, it is supposed to be tapped into a switched power source, so its requirements are minimal.