Depends on length of run and what you're hanging off of it. For charging and those loads you list 4 AWG cable is probably sufficient.
2AWG us about 0.00051 ohms per foot
4AWG is about 0.00082 ohms per foot
So if the current on the cable is 50 amps and the run is 20 feet, you can find the voltage drop (0.82V for #4 and 0.51V for #2). Significant but not earth shattering.
If you're charging a very flat house battery and all the charging current can go to it (e.g. the starting battery is either switched out or not drawing any), then #2 will start to look more necessary. IOW, 100A at 20' gives you 1.64V vs. 1.02V. That means if the forcing voltage is 14.4V but the battery is seeing 12.76V compared to 13.38V. The difference here is that the battery won't actually charge much at 12.8V while it will at 13.4V.
The larger issue is in this example the #4 would dissipate 164W while the #2 dissipates 102 watts. That's a sizable difference in how warm the cable will get.
'91 Toyota Pickup
'09 Kawasaki KLR650
'12 Gunnar Rockhound 29
"To live is to suffer, to survive is to find some meaning in the suffering." -- Friedrich Nietzsche