Joe,
Minimium size is no hatch cover. Use a sea sock instead and load through your cockpit.

Intial European standard was 9 inches round. The reason dates back to WWII when kayaks were used for special forces insertion. The 9 inches ensured that the number 9 k-rations can fit in the hatch.

Largest? Well Seems to me as long as structural integrety is not compromised the skys the limt. I have a friend who has a 37 foot kayak 54 inch beam. He and his wife sleep below deck. So the hatch fits both of them. I know may folks who mae their hatch fit ther sleeping bag.

It is more common to have a smaller hatch and to pack smaller pieces through it. Many production boats however are going nearly sheerline to sheerline. I'm not fond of hte look but I have sure enjoyed the ease of packing and finding with a big hatch.

Personally I like a long thin hatch to one side long enough for my tent poles.
But everyone has a different view on that. It comes down to how you like to pack
and what kind of trips you take.
Basically its personal choice

!RUSS