I don't think you really will know for sure until you're well into your twenties.

Secondly, 5.5 inches is not pretty small. It's actually PERFECTLY average.
Thirdly, you're only 17. A male's genitals can continue to grow until well into his twenties. But as I said, you're the average size (which, statistically is actually in the 5.5 inch range). So, really, you shouldn't be ashamed/shy/uneasy about your penis size.

Superficial movies and porn lead us all to believe that 5.5 inches is small. They're lying lie-face liars.