need suggestion about big keyword or small keyword

when i post the product, should i write the big keyword or small keyword to describe my product?
for example, if i produce one metal ball .
if i wrote balls, someone who need the balls may search my website, but maybe may balls is not he wanted. maybe he need the plastic ball.
So even though can bring more traffic but it is uselss.
If i wrote metal ball, the traffice will be less, but once they searched the metal ball, they mostly want to buy metal ball.
So what is your idea about the keywords?
Could i get your advice?
I should write big or small( exact to describe my product) keywords?

Thanks.

It's really hard to understand what you're asking.

It seems though, that you answered your own question. Is it better to have more traffic that will leave your site straight away, or have less traffic that will stay on your site? Go with the second, it's better for your bounce rate.

I am so sorry let you can not understand me easily.
Yes, you said right.
My question is i need write a big range keyword to describe my product to get more traffic or write an exact keyword which is related with my product to get valued clients?

Thank you.

Again, as iJonnyx said, we suggest you should go for the "exact keyword which is related to your product", because yes: this is going to get you valued clients.
Traffic is worth close to nothing if achieved by a wide range (that's what we call: generic) keyword when users bounce off your website. Low conversion rates and high bounce rates can affect your rankings. If you're selling something online, it pays off to go for the "low range" (that's what we call: longtail) keyword.
(Just a side note: There are a few beginner's tutorials right at the top of the thread list in this forum. They might provide you great help, especially for any misunderstandings. If you use your own terminology, people might just have difficulties understanding your question, or you might get the wrong answers because people got you wrong Good luck )

You have given the answer to your own question. Yes definitely, you should choose exact keyword for what you are selling. otherwise useless traffic will hit your website.

Each of your pages should target your specific keyword. You need to do keyword research and pick easy to rank ones. So if you target is round silver metal balls, your pages should target that exactly.