To start with, you want something with a lot of floatation which will give you stability and make it easy for you to catch waves on! I don’t know how heavy you are but with have great results by starting students on 8ft foam boards.
The volume massively depends on how much you surf and the type of waves you’re surfing. You don’t need to get too fixated on the volume at the level you are at. Ride a board that is going to enable you to catch a lot of waves and start to turn on. When you get to the point where you want to turn more and the board is not enabling you to do so, then it’s time to move down to a smaller board.
If you are not catching most waves that you paddle for, you need a board with more volume.
You can type in google a surfboard volume calculator to give a rough idea but it’s just a rough idea!