First you have find the risk amount you want to use in per trade . In generally needs to risk 1% per trade so i am assuming that you have $266 dollar balance . So if you take 1% risk in per trade it would be $2.66 risk in per trade .
So to calculate position size you have to follow the formula below . If you have micro account you need to first find the tick volume of per trade . In case of micro account it is 10 for per volume.
Position size = (1% / (stoploss * tick volume)) * 1000
In my case i always add 2 with tick volume . If here my stop loss is 200 points then position size will be
Position size : (2.66/(200*12)) *1000 = $1.10 dollar is my position size .