The smallest amount I've invested is $1,000 which I held for about 2 months for a return of about 15%.
I know that's not a huge return compared to what others achieved, but I'm still new to this (Started learning about the sharemarket about 8 months ago).
On the other hand I've invested $1,500 in a couple of trades which have barely gone up by 1% in the last 4 months.
I guess it comes down to your style of investing and experience. If you're holding for a while, there's no reason you can't invest as little as 500 - you just need to work out how much your total brokerage is and then see how much of a return you would need to cover brokerage and return a profit.
Let's say your buy sell brokerage is $30 - therefore your total trade will be $60 (30 for when you buy, and 30 for when you sell)
If you were to invest say $500, you would need to sell at a minimum gain of 12% just to cover brokerage costs of $60. Let's say you get a gain of 20% - this will yield you $600, which, less brokerage, leaves you with $40 profit (pre CGT tax).
Not a large sum of money true, but look at the gain needed to get it: 20%. So you need to ask yourself if, based upon your experience, trading strategy, timeframe and risk management, can you get a 20% return or greater in order to make the trade worthwhile? (NOTE: This 20% gain relates to the $500 example only)
So you need to sit down and have a look at what is an acceptable return for you, given your personal circumstances and you can then determine what is the best minimum for you.
Either way, even a small amount is beneficial as you'll learn a lot about the market just by 'having your skin in the game'. Even if you lose it all (i hope you don't) the lessons you'll learn from it will be invaluable.
Best of luck.