disclosure: idk how electricity works.
calculations
W = A * V
V = W / A
A = W / V
sizing wires
tldr:
size wires so that the max current/shortcircuit current (Isc), is no more than 75% of the wire's ampacity (maximum currentcarrying capacity).
this is the safety margin recommended by the National Electrical Code (NEC)
example: if shortcircuit current (Isc) is 10a, you'd multiply by 1.25, giving you 12.5 Amps. So, you would need a wire that has an ampacity of at least 12.5 Amps
the longer the wire is, the more it "bleeds" voltage, which is called "voltage drop"  use a voltage drop calculator to check on this. making the wire larger will reduce the voltage drop.
sizing fuses
for dc: appliance wattage / 12v * 1.25 = fuse size

select a fuse larger than the appliance's maximum draw

do not exceed the current rating of the wire
series vs parallel wiring
series wiring connects components in a sequence, one after the other, so the same current flows through all of them. if one part breaks, the circuit stops working. in series circuits, voltage is summed & current is the same
parallel wiring connects components along separate paths. each part receives the same voltage but they can have different currents. if one part breaks, the others can still function. in parallel circuits, current is summed & voltage is the same
for solar, parallel wiring is almost always better bc then shade can't annihilate your circuit. but it makes wiring a lot more complicated, since it requires more fuses & breakers, and larger AWG of wire. it also requires a combiner box.
as usual, i've opted for a slightly less efficient system, leaning in favor of simplicity  hence, I wired my panels in series.