When you put in the HO alternator, you will want the larger leads anyhow. I bought a batch of 2/0 welder cable and used that for battery, starter, alternator, and main power distribution (I have some extras installed) as I am a firm believer in overkill.
OEM speaker wire is usually about 20/2, with performance wiring running to 16/2 or (sometimes) 14/2. Anything bigger is pretty rare, and is not really required except for really L-O-N-G runs or exotic high-powered amplifiers - and you would put a high-powered amp as close to the speaker as possible to take advantage of the inbuilt filter circuits in the amp anyhow.
Unless you're running more than 500W per speaker, I don't see much reason to go past 16/2 wire for the install. It helps that "speaker level" outputs seems to run about 3-5V, as I recall...
5-90