While it's reasonable for a job applicant to expect a stable well-paying job, the question usually indicates to me that the person is clueless about startups, and shouldn't be working there.
There is no such thing as job stability anymore - look at big investment banks on Wall Street, most of them lay off 15% of their underperformers every year; or even more than that when they fail to meet investor expectations. We've seen huge companies like Enron or Tyco lay off thousands of hard-working employees, because of mismanagement and fraud.
At the same time, we all know stories of 3 people working in a garage, becoming millionaires because they were passionate about what they did, and worked to succeed against all odds.
How do you know the small company is stable? You don't. If you underperform, you'll be fired. If the company loses money, then you'll lose your job.
But if you are really good - you will easily find another job, and will gain invaluable experience about what went wrong in a business that you worked at.
Just my 2 cents.