it really depends on how much money you have in your pocket or wallet, may be big or small, as long as it does not belong to your savings, you should invest money that you are willing to take risk, because crypto trading is not that unstable in terms of price because of the volatility of its market price, some even invest their money that came from loan, if you are well enough to trade and knows the ins and out, you can invest as much as you can, but it is still best to invest money that you are willing to loose in case of a dump.