Whatever percentage you plan to use. 3% is the one I see most commonly. 5% if you want to be aggressive, or if the amount in your account is small (relative to you). 2% or even 1% the larger your account gets?
Let’s pick 3% for right now. If you have 5000, then 5000*3% = $150. So you know that you can risk $150 per trade.
As a setup appears, determine how much stop you need. This varies on every trade with Cowabunga, because it is the last swing low/high, whatever that was. As the crossover is nearing, check to see the last swing low/high, and compare that to the current price. Whatever the pips of stop is for that trade, divide the 150 by that.
If that amount on the current trade is 35 pips, then you divide 150/35 = 4.28. That means you can go 4.28 minilots (or .428 standard lots).
If you have $3500 in your account, and the last swing low/high was 57 pips from the current price right as the crossover candle is closing, then $3500*3% = $105/57 pips = 1.84. So you can go 1.84 minilots on that trade or .184 standard lots.
Same $3500 account, but say the last swing low/high was only 17 pips, then $3500*3% = $105/17 pips = 6.17. So on this trade, you can put 6.17 minilots or .617 standard lots.
The targets on Cowabunga trades also vary, one with only 17 pips stop can often have a lesser target than one with a 57 pip stop. Granted, you can sometimes catch 100+ pip runs with Cowabunga, and if that happens on a trade you have 6.17 minilots on it, that’s a lot nicer than if it happens on one where you have 1.84 minilots on it.
To adhere 3% risk per trade strictly, some are going to be bigger than others with the Cowabunga system.
Hope this helps.
Jeff