Lot increase will always going to depends on your strategy and your capital. Because if you going to double your lot size if your capital is not high enough to sustain your position then you can end up losing money instead of making money. In forex trading its all about sustaining your capital. Also you can trade not only double your lot size but even have your lot size increase by one decimal or even more. As long your capital can sustain your position theirs no limits on how big your lot size is.