It depends what you're looking to do. If you're looking to make a living, then you probably want 100,000 dollars or more. If you're looking to speculate and learn in the markets and don't have a great deal of experience, I'd recommend 10,000, or even going to Optionshouse, and opening a 5,000 dollar account. The benefits of a 10,000 dollar account is that you can take positions while practicing a certain basic balance of risk control in the futures market and money management which is the key to success in the markets. You can practice money-management witch 5,000 dollars you're left trading some Agricultural Mini futures, and Bond Market Financials, but it can work.
I started with 10,000 dollars cause I didn't know much about Futures but was interested in the leverage, I lost 4,000 dollars swing trading and day trading them in the first 2 months. I did have a great success that year, and the years that followed, and I'm having a great success this year. I'm doing this cause I got out of my Brexit recovery trades last week, up 264% for the last 3 months, and the wife wanted a vacation and didn't want me trading.