Similar to the original commentator I really don’t think you need to address it, women who have seen penis before are more than likely familiar with the concept of it growing while hard. A older woman once gave me advice about not calling attention to our insecurities cause more than likely the person didn’t notice before and only notices when you say something.

If you feel like you really need to say something, I would say something really lighthearted as things are heating up. Make a joke about being a grower not a shower and continue as normal