Start with a conversation, not a number
Volume is one part of board choice. Length, width, outline and construction also change how a board feels. Tell your instructor or rental provider where you plan to surf and what you can do independently. Ask them to explain the choice in terms you can test in a session.
Rent before making a large purchase
A few supervised sessions on appropriate rental boards can help you compare stability, paddling and handling. Keep notes about the whole experience, including carrying the board and controlling it in the shallows. A board that looks easy on a shop floor may not suit your transport or storage.
Inspect the practical details
Check the leash and fittings with the provider and ask how to handle the board around other people. If renting, record existing damage and agree the return procedure. If buying used, ask an experienced repairer or instructor to help you assess condition rather than trusting an attractive photograph.
Change equipment for a reason
Move to a different board when you can explain the problem it solves. If catching waves or positioning remains inconsistent, a smaller board may add difficulty without helping the skill you want. Use coaching feedback and repeated sessions to guide the decision.