You should only risk a small percentage of your total account balance on each trade.
This simply minimizes your risk, so that even if you end up losing your entire investment
on a trade, it doesn't have a critical effect on your account balance. The recommended
amount is 25% of your account balance per trade. More aggressive traders go as high
as %50, but never higher than that. It is a very important rule to keep, since the
lower your account balance drops, the harder it is to rebuild it.
Another consideration to be examined, regarding the suggestion to open small trades,
is that in the case of a losing trade, you can then open the opposite trade with
a bigger investment or higher leverage, thus compensating for losses.