I would say start with a 0.5 BTC bankroll, but trade with small stakes, like 0.005 or 0.01. This way you will feel comfortable with your trading stakes and will probably not get emotions get in your way. If you are a real beginner, try "paper trade". Meaning you should simulate your trades to test your strategy, and commit to it, with no emotions attached to it, because if there is no money involved then there is no chasing losses etc. You you feel you have a good strategy, try it out with low stakes, and start increasing your bankroll as you progress.