I can't answer your RCA question EXACTLY...since I didn't use any...you see, I still have the stock headunit in my car....that amp has speaker level inputs....so its all done with speaker wires, no rca's....
BUT, if you have a head unit that has rca outs...then I would get a set at least 8' long to go from the headunit, down through the console and under the seat to an amp.
as for the size of speaker wires....its not that criticle...seeing as how I stuck with the stock wires from the dash out to the doors...and if you look at them they are pretty small...but beleive it or not they are just fine up to about 50-75 watts...and I am only pushing 40 watts/channel......the speaker wires I used to run from the headunit to the amp and back to the dash was the speaker wires that came with my new speakers....I think they are 16 ga. maybe 14....
3 meter cables should be enough but if you might ever want to upgrade and install an amp in the back you will be SOL with 3 meters so i always get 5 meter.
