What is a decent amount of Bitcoin you should have to make day trading worth while?
it's totally up to you. low invest means low profit and also low risk. so i think it is wise to start with low amount for beginners so when you feel confident then you can increase your amount.
But low profit isn't worth with your time , talents and efforts you did spend on it, right?
Why not keep holding your bitcoins, go and do the research on your own. I always prefer reinvesting into potential altcoins rather than trading.
Keep on investing with good altcoins instead of doing trade because some altcoins will pump higher in 2018. Trading is very risky, if you want to earn more money through that then you should spend a lot of time and effort for you to succeed, and also a lot of patient.