Impossible to tell you. Since you're the man on the spot you'll have to make that decision. Regardless of how big, or small, you make it you will have to determine the steps needed to make it strong enough. This may entail adding more cloth, stiffeners, lips, struts, etc. When I add large hatches I usually make raised hatches so the lip (and fillet) will add strength. I also extend the lip down inside about a 1/4 to 3/8 inch and fillet that. Flush hatches are a different type of problem. I have no shame when it comes to adding hidden, internal, struts and lips for strength. I would think the plastic hatches are on the small side, but if not, and if necessary, you can still add more cloth, lips, fillets, or some other ingenious stiffener, in strategic locations to give the strength needed. 'course...now that I've said all this somebody will probably come up with the exact size and method you need.