In order to calculate the Volume of the wetland:
Using the 5 day recommendation: Required Volume of wetland = 5 days X Volume of Greywater generated per day.
Assuming you will generate 100 L/day of greywater, the required volume would be 100L/day X 5 Days = 500 L.
Technically, in order to achieve 5 days of retention within the wetland you would need to factor in the volume occupied by the gravel, which would result in a design volume of 2 to 3 times larger, depending on the characteristics. For your application with a final discharge is to a swale system, I think this would result in something unnecessarily large.
A few considerations if you choose to incorporate the constructed wetland as part of your system:
If kitchen wastewater will be part of your greywater, I would recommend using a woodchip filter prior to the wetland to remove any food particles, oils, greases etc. so you don't end up plugging the system.
protect the inlet and outlet piping and ideally allow for inspection. Using
milk crates or upside down buckets with holes drilled in them surrounded by larger rocks than your gravel material will protect these critical areas from clogging and becoming matted with
roots as well as facilitating inspection and maintenance.
For the gravel material, if you have pea gravel, that is what is typically recommended, but if you have something else available you can use it, best to wash it well prior to use.
The typical recommended depth of a constructed wetland is usually around 0.5 m, so maybe you could use half of your IBC as the wetland reservoir, but any other water tight vessel or liner could also be used.