I have a Roleplay setting on the books. It's a constructed pocket universe that the Dragons built to hide from the Great Old Ones; because of its structure, there are vast bodies of water, potentially millions of kilometres, of deep, open ocean water without so much as a speck of land. Such expanses are generally considered by those who live on their shores to be effectively infinite since even with a healthy dose of magic the distances involved are just too great.
Over millions of kilometres, without any land-based support and resupply, any ship will eventually founder. But what I'm wondering is how narrow an ocean might be and still be effectively infinite to a sail-based maritime civilisation. Human civilisation is rather variable in this world so as the technological benchmark for creating a relevant answer use the height of the Age of Sail and ships like the Cutty Sark or possibly a sturdier version of the Wyoming; iron or steel framed but wooden hulled vessels in the 90-150 metre range with displacements up to 10,000 metric tonnes. My question is what would be the maximum range of such a vessel that was built specifically to go as far as possible, and conversely, how large would an ocean have to be to defy effective exploration by such a vessel?
Please note that while the world contains magic I have deliberately left that tag off with the intention of excluding it from consideration within this particular question, with one exception; a ship's crew can use magic to evacuate if they cannot find land so exploration trips can be made to the absolute limits of supply endurance.