This information has not been confirmed by Square Enix yet, but PlayStation Game Size has a good track record for such details. Therefore, it seems like Rebirth will indeed occupy a significant amount of space on your PS5.
This is further supported by the fact that Rebirth needs a separate disk for installation, indicating its large size. The reasons why other games do not require a second disk are unclear.
It’s worth noting that the mentioned size is for the day-one installation, and it may increase with patches, updates, and possible DLC in the future.