You can figure the amp draw from everything and add it up, to figure the size of the alternator, battery is to start your car alternator powers the electronics.
Stereo alone can eat a lot of juice, 1000 Watts rms / 12 = 83.33 amps obviously that's cranked and at music peaks.
To figure out the amps of each fused circuit fuse size is the max value of those circuits with a safety margin.
Obviously everything generally ain't on and going full tilt at the same time so you don't necessarily need 100% capability.
