You don't need to know how big a lot is.
AccountBalance * percent = Risk = ( OrderOpenPrice - OrderStopLoss )*DIR * DeltaPerLot * lotsize. Solve for lotsize and normalize properly.
You place the stop where it needs to be - where the reason for the trade is no longer valid. E.g. trading a support bounce the stop goes below the support. Note that OrderOpenPrice - OrderStopLoss includes the spread.