How do you know how much penny stock you can trade without changing the price?

[john]

I realize of course that buying stocks that are trading below $1 is very risky business. But I also realize that a major reason for this risk is that trading volume on these stocks is so low that even trying to sell a small amount of shares with cause the price to plummet. What information (on Yahoo or Google Finance) can I look for to give me an idea of how many shares can be traded? Is it volume? Or something else. And when I look at this information, what types of numbers should I be looking for?