At least 14028 square miles
Let's assume the Dragons are scared and Over-engineer to avoid detection and believe that the crew is perfectly optimized with perfect food storage
Crew Size
For a ship like the Wyoming, a crew of at least thirteen people needed to be working for its operation.
Space
From the wiki, we learn that
a cargo capacity of 303,621 cubic feet (8,597.6 m3)
this means every person gets provisions for (8,597.6/13) -> 661.4 m3 of supplies
Food and Water
According to this site a sailor will need about 3.7 liters of water or 125 ounces per day or (0.0037 m3).
Common knowledge on the back of any food product will tell us the recommended amount of food is 2000 Calories.
If we are eating a balanced diet and proper portions
- 5 1/2 ounces of lean meat - roughly 1 cup
- 2 1/2 cups of vegetables
- 2 cups of fruit
- 3 cups of low-fat or fat-free milk or the equivalent dairy products
- 3+ ounces of whole-grain products - roughly 3 cups
10 cups of food or (0.000236588 cups in a meter3) 0.00236588m3
which ensures people consume (0.0037 + 0.00236588)-> 0.00606588m3 of food and water per day.
since we have 661.4m3 of room for supplies, we divide by 0.00606588m3 to get around 109036 days of travel before
Speed
How fast can our ship go will determine how big the ocean must be. If we look at the upper limit of current technology, it seems that the fastest we can go is around 64.45 knots or 75.2 miles (a world record but seems achievable with magic).
Assuming tireless workers, that is (75.2*24) = 1804.8 miles in a day that could be traveled
Total Travel Distance
1804.8 * 109036 => 196788172.8 miles -> before you run out of fuel and need to teleport back outside of pocket realm
square root of 196788172.8 is 14028.1207865 which is the area you would need