There are a few different options for position sizing.
One can risk a fixed fraction on each trade....i.e. the 2% solution. One can also trade this method based upon acct size, that is trade 1 contract for every 10k in the account.
If one wanted to maximize growth, you should study the concept of optimal f. This is the % of capital to risk on every trade that will maximize the growth of the account. The downside is that the drawdowns can be severe. This strategy considers the largest losing trade, and uses that to determine how much to bet on each trade.
An alternate version, is to consider drawdown, instead of largest losing trade. Slightly more conservative, the drawdowns will still be very large.
The latest and greatest on the optimal f front, is the concept of scenario spectrums. That is creating your own scenarios for the possible outcomes on a given trade, accounting for as many one can imagine, and bet the optimal amount based upon your forecasted results. (This sounds wonderful in theory, but I have yet to be able to determine the outcome of any of my trades in advance, and assigning probabilities to outcomes seems more like voodoo, then anything else!) If you're an optimist like me, you'll likely skew the scenarios in a positive direction, and if you're a pessimist you'll skew them in a negative direction. Neither is optimal, but if one has to error, it is better to error on the side of caution, as there is no benefit to be gained by trading more than the optimal number of contracts on any trade.
Then there is the fixed ratio method, pioneered (if you could call it that...I'm sure traders were using a similar method years before him!) by Ryan Jones. This method requires one to increase contracts at a fixed ratio. If it takes 10k of profits to increase to two contracts, then trading two contracts will require one to earn 20k of profits to trade 3 contracts and then continue from there.
Last, the method Williams writes about, is this. Decide on an amount you are willing to risk on a given trade, for instance, 5%, 10%, 12%, maybe even 18%. Then take your account balance and multiply it by your risk %, then divide that by the largest loss, and that's how many contracts you'll trade. The key to this strategy is to use the actual largest loss, or your perceived largest loss.
There you have it, a summary of methods for position sizing and risk management. Set up a couple of spreadsheets, study and really know your strategy, it's strengths and weaknesses, and then figure out which method will work best for you.