Power line fuse is designed to protect the battery and car fire.
The fuse per IASCA rule must be installed no more than 12" from the battery terminals since it is unlikely anything might cause a short within that range. In the event of an accident, overload, short to ground (as in your case), the fuse should blow. The amperage has to do with the design for the TOTALITY of the load. The load (amperage) effect the wire gauge (size) and at a given length. Proper fuse sizes are dependent on the amount of amperage drawn by the amplifier(s). Wire size are not only dependent on the amperage drawn but also the length run. Typical for the main power cable is about 20' in an average car. So for a 50 to 70 amp fuse, you would need to have a 4 gauge wire. Higher gauge (2, 1, 0. etc) can be use but not necessary given the length run and amperage drawn.
To break it down in simple terms, a fuse is merely a conduit that is designed to break should the load is above rated draw that can be handled by the wire. It is there to protect the cable from overheating and melting the sheath thus causing a fire. So the fuse amperage should never be more than the design of the wire's capability. So running a 70 amp fuse on a 16 gauge wire will turn your 16 gauge wire into a heating coil in the event of a short because the 16 gauge wire will melt before the fuse does.
The fact that your car did not start might be trace to the poor install but I would consider resetting everything before deeming the "BCM fried". Disconnect the power cable to the amps at the battery. Double check the battery for damage, in-fact use a new battery if possible. Re-sett the car and go from there. I doubt it caused that much problem since the short if causes by the power cables grounding to chasis would be a closed system and should not have effected the car's BCM.
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