Substitute for Fees – Are RBF transactions on average larger in size than previous and non-RBF transactions?

An RBF replacement transaction can have a size that is larger, smaller, or the same as the transaction it replaces. Basically it depends on whether you are adding or removing inputs and outputs.

• If the original transaction had a change output and the output amount was reduced, the replacement transaction will have exactly the same size.
• A replacement transaction becomes larger in size if it adds more inputs than if it adds or removes another input to raise a larger fee. At least one input must be identical. Otherwise, previous transactions will not be replaced.
• A replacement transaction adds an input large enough to remove more inputs than were added, or becomes smaller if the input amounts are appropriately summed to remove the change output.

I don’t have the data, but I would guess that most replacement transactions fall into the first category, with more transactions adding inputs than removing inputs, making replacement transactions slightly larger on average.

How many bytes does the opt-in RBF signal contain?

Opt-in RBF sends signals through a sequence field that is part of every input, so the signals do not take up additional space in the transaction.