Welcome to the forum!
The Roger Russell site that Rick provided a link to is my favorite, and I use the table that isn't too far down on the page that shows recommended wire gauge versus wire distance. According to the owner's manual for your Yamaha system, the speakers are rated at 6 ohms nominal.
While the wires that come with those systems seems pretty thin, I'm guessing that you won't notice a difference if you upgrade it. However, if you're like me...you won't be happy until you find out.
Like Mark said, you can get inexpensive speaker wires. You can pay more for aesthetics, though, which I have done.