Everything, nothing, or some. Depends on your system.
If you don't know, your system is not complete, let alone tested. Scaling, and size management is an integral part of a systems risk management, which in turns is part of tactical execution, entry and exit.
It is not an afterthought. It is not an "out of plan" rescue method. Either embrace it with a well though out and tested plan, or don't. Going back and forth to avoid loss management, is like changing time frames after a trade goes wrong, or making a trade into an investment once it goes sour.
It is another tool. Use it wisely and prudently and it can be good. Don't respect it and it can close accounts.