How I select the  position size either in value or in quantity.

I suggest an exit alternative using the price of n maximum or n minimum, for example, when the price breaks the maximum of n days, the exit price will be the maximum of n days yesterday.
example:  today MAX is $ 10,60
the indicator MAX(5 days previous high) is $ 10,50
the price sell used in this  case will be $ 10,50 and not open, close, low, high of today.