Though your questions is simple, it is not a simple answer. I think a lot depends on your style of trading. Whether you scale into positions or not, are you a swing trader, day trader, position trader etc.
I am scalper, so for me it is 100% unless I go on a big draw down like 20 ticks, then I cut my size, until I figure out what I am doing wrong. However even though I trade a 100%, I am usually only risking on 2 to 3 ticks, so from a $$$ management perspective that is about 3 to 5 % of my capitol.
Things you need to do find out to arrive at a figure
1. Which contract - will determine margin
2. How many contract - Will determine your account size
3. Your Broker - Will determine cost of taking a trade
4. Cost of Charting, Data Feed, Platform, Renting Desk.
5. How much time in month you are willing to commit - Calculate your food, rent , internet, movies, cloths, holidays, books etc.
After calculating above things you need to determine your account currency. If you are trading Eurex fund in Euro, If CME fund in Dollar so that you are covered for exchange risk. - Not very imp.
Now you have to start trading for at least a month with a sizeable no of trades to get statistics of your trading based on which you can refine the 1-5 parameters above. Its a continuous process. - Very Imp to get a large sample of your trades without blowing out to give yourself a 2nd chance.
After trading for 3 months usually people change there broker for low commission, trading platform to match their trading style, product to get them max advantage for succeeding. So its a continuous process and 1st step is to calculate 1-5 and start like Chinese said way to a 1000 miles begins with a small step.
Over analysis is paralysis the more you think about the unnecessary things the more time wasted and opportunity lost. As a new trader first thing is to fund the account and start trading. Start with a small contract like M6E where per tick is around $1.25. and commission is low. - Right Product selection key to match the product with your style.
My trading style is between a Day Trading and a Position Trading (from 1 day to 1 year, on average 1 week). I invest on Futures in CME, CBOT and Eurex not more than 40% of the entire capital, the other 60% is cash for margins variations and losses. I wonder if 40% is the right thing. I would like to know experiences of other Traders with my kind of trading style