I start trading from about 0.005 BTC, it was because I was totally new to trade, and I want to invest a lowest amount so that I will not lose any bigger amount, after that I remained successful and I increased my amount of trade , so I continued in this way like increasing my amount of trade and at the same time I continued to learn.
0.005 now is enough to actually start with but most trading websites charge a high fee which makes the profit almost 0.
To start, it's fine but then the minimum amount we should choose is 0.1 so that we can earn atleast $5-$7 profit every week. Risk factor also exists as some trading websites shut down with out money.