Looking Wire a generator 4 #2's and 4 #18's what size PVC pipe is required ?

What are the #18 wires for?

Two communication and two for battery charger

#18 is not allowed to be used as a building wire, the charging circuit is 120 volts and will need to be a min of #14.

The communcation cannot be in the same conduit as the power.

TK, many generators, such as Generac, do allow the control wiring in the same conduit provided it is a limited length of run.
The control wiring is typically a 240v reference signal that tells the genny that the utility is on, and the other is the load side of a battery charge circuit coming from the transfer switch.

I will say though, #18ga seems small to me. I would not use smaller than #14.

Control is one thing, communication is another. All I can go by is the information provided.

Perhaps Wired1 can provide more details of what is being done.