The wire size depends on the amperage maximum draw of each of the components, the temperature rating of the wire itself and the distance it will be running. there are many charts available online, here is a simple one that is kind of "corrected" for 12 volts DC. Please note that I am not the author of this chart, there are actual NEC and Automotive approved charts - and this is NOT one of them !
Gauge 110V 12V
22 5A 5A
20 7.5A 8A
18 10A 10A
16 13A 20A
14 17A 40A
12 23A 60A
10 33A 100A
8 46A 150A
This is assuming common (cheap) automotive "hook up" wire which is typically rated at only 75 degrees C or at best 90 degrees C (we produce wire that is rated at 200 deg and even 250 deg C, it makes a huge difference)
If you do not know the max amp draw of your pump or fan, there are 2 solutions, put a common automotive amp meter in line with the device and power it up, watch it when it starts (it is when the amps are max) and this will give you the amp draw. Add a little resistance if you like to see the amps go up - "don't get caught up in the fan !!!" (I know you have an old amp meter banging around in your toolbox). Add all the amp draws together, and there you go.
another method is if you know the power of the device (for example; 750 watts) Current (I) in AMPS is equal to Power (P) in WATTS divided by Voltage (V) applied in VOLTS - or more simply put in "Ohm's Law" I=P/V
Always "round up" to the next size wire, especially if it is a long run or if the temperature rating of the wire is unknown (or you know it is cheap wire, lol)
Hope this helps.
JD