I don't think it's important. If you only have a few hundred dollars, you can participate in transactions, but then your profits will be lower, and it's better to start with a few thousand dollars.
Yeah i think in any investment business they are risks involved, so the best figure to start with is the one that don't make you nervous and you are comfortable with losing it if things go south. If you risk money that are important to you, it's very easily to make mistake when market correction arise. So be comfortable with the numbers and do not make scary decisions.