how to double the lot size after the previous negative close position.

Double lot size on what type of trading?....i.e. Binary, Traders/Dukat Contest, etc??

You can use the blocks "Calculation" or "Calcultion Expression" to increase the lot size mathematically.

Hi;

First you need to check the P&L of the position closed: If <0 then you move forward:
Take a calculation block and drag the position amount used in it and multiply it then use the same position amount variable in the result so that it will be used in the next flow.
Make sure this calculation block is executed only one time otherwise you'll find yourself with a huge positions

Cheers