Repeat after me:
Fuses are NOT to protect the device they supply power to!!!
They are there to protect the WIRE that feeds the device!
To determine fuse and wire sizes you must first know the max current a device will draw AND the distance from the fuse to the device being protected.
Example: You have a light bar with two Hella spot lights on it that draws 6 amps total when on and it is 10' away from the fuse panel. What size wire and fuse would you use for this circuit while not allowing voltage to drop more than 3%? (Voltage drop affects light output!)
Answer: 16ga wire and a 15amp fuse. Why? 10' of 16ga wire will actually handle 25 amps of current but you don't fuse for the max allowed, you fuse to keep the wire from overheating if a short occurs so 15 Amps is what you choose.
Blue Sea has one of the best calculators out there for determining this stuff. You input the circuit voltage, load current, and distance from fuse to device. Also enter in the battey CCA and if the panel is the main fuse panel, or a branch panel, and it will tell you not only the wire gauge to use but what size fuse is needed.
Circuit Wizard - Blue Sea Systems
You can also get this as an app for your phone...
Now that being said: You need to add up all the device currents that will be connected to this 100 amp rated panel. If the sum is greater than 100 amps, you need a bigger panel. Another factor to consider is that you will not have all devices on at the same time. This means you can have 60 amps of device draw on a 30 amp fused circuit as long as you do not have all devices on at the same time. OEM Toyota fuse panels usually have 1 to 3 devices on one 15 amp fuse. If you added them all up it will be over 15 amps but the odds are you will not have all devices on at the same time. Even then, fuses will handle their rated current for a period of time before they blow allowing for brief periods of "overload".
Example: You have one slot left in a fuse panel. You have two devices that each pull 6 amps EACH when on. Both devices MIGHT be one at one time. What size wire for EACH device, and what size fuse for BOTH devices do you use?
Answer: 16ga wire for EACH device from the fuse panel to the device, and a 20 amp fuse. (A 15 Amp fuse can be used if the devices are turned on one at a time. )
So find out the current draw of each device
Determine how far from the panel the device will be
Go to the link above and calculate the wire size needed for that run of wire and the fuse size needed to connect it.