Money management suggests (by most of the experienced traders) that we should take 2% to 5% risk in every single trade. Anything over it means we are risking it too much. Here are two other things to be added that are risk-reward ratio and winning percentage. We should maintain at least 1:1 risk reward ratio with at least 60 winning percentage.
I think it good to keep the risk about 2% or 3%, expert traders always suggested to trade low risk and safely to limit the risk only 2%, in practice the hard part is how to control emotion and keep trade based risk management level, maybe use expert advisor will help in trading.
If we talk about money management principles and you are interested to know about the percentage of your deposit to invest in the deals, then I would say that the most adequate number wuld be around 3-5% depending on your trading style and risk portfolio. However, this number is relevant to total amount of opened deals but not on one single deal. It is very important to remember because if you open 10 deals with 3% risk, then the totak risk proportion will be too high. I don't think that it would be a good idea to put at risk more than 5% of your total budget because it may destroy your psychology and, consequently, your whole budget. Anyway, every idea should be tested on practice and it is not clever enough to theoritize such things.
All traders have their individual risk percent, becaue risk management practices are also very individual. Some traders prefer to comply with general rules like open position with no more than 3-5% from your total deposit, while others traders got used to open positions with all their deposit plus they use leverages. Here everything depends on how much risks you are ready to take on yourself. In my opinion, if you're a novice you should comply with general rules, but when you will get enough experience, you will be ready to decide on your own.
Moreover, remember that the higher risk the more money you can lose, and of course, the more money you can earn.