You could use an extension cord, string, rope, christmas lights, shave off and tie your pubes together. There are all sort of things you can use to figure out the length of your RCA cables. You can figure that out, because it won't be exact, and an extra foot or two is usually best for pulling the head unit and installing. You're not going to get exact. No need to add another week to down time for that.
Speaker wire is cheap. I would buy a 50ft roll and call it a day. I also wouldn't buy two different sizes, 12awg and 14awg. Just get 12, and save yourself the hassle, and probably a little money, as opposed to having two sizes of leftover scrap wire. You either have one size, or you have one long enough for a run.