Plus not More that you can afford to loss.
I think this must be the measure to make investment as a newbie in any market. First you have to get all the required knowledge for the investing and the techniques one must have to not to be in loss. Then you only invest that amount in start that you can easily bear to loss. If you can afford a big to get profit and loss too, invest it.
Setting up minimum investment based on affordable limit to lose must be right idea but in practical while trading there will be less chances to lose all our capital because on the occurrence of negative market moment, we will lose only some percentage of our capital not all our bankroll like what we are facing with gambling. So, the minimum investment does not need to be exact amount of what we can afford to lose, it can be double or triple the amount of what we can afford to lose.
After this, the minimum capital requirement could be set based on how much profit we we may expect. Usually when trader is setting up this limit, their greedy will come to play. But a limit for our profit levels should be set based on our skill levels.