It is not an exact science - judging by the boat size you need a zinc in the vicinity of 3 pounds.
Do you have bolts on the hull for the zinc? pay attention to the distance between them to ensure that the new unit can be attached.
The attached picture shows the zinc that lives on my boat (L400) doing its job very well. It is 1.2Kg = 2.6Lb. You will need an unit somewhat larger.