The size of any single ocean is limited by the maximum size of a biome, meaning oceans in the region of ten thousand blocks wide and long ( source ).
If you've travelled 15km North-East, you should be arriving at land soon, unless...
If you include the possibility that an ocean biome can be placed next to an ocean biome, then you could have any sized ocean.
In theory, one could imagine a seed that created nothing but ocean biomes, resulting in an entire map of ocean.
This is pretty unlikely though.
Of course, in newer versions if you choose the 'Large biomes' option when creating your world the oceans will be much larger still!
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They are huge , absolutely humongous.
In this Tectonicus render you see my map after I have traveled for about 10 in-game days (I think) in one direction, got fed up, changed direction, got fed up again and headed back home.
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30,000 blocks +
In 1.8 I started walking from the spawn point towards positive X, positive Z. At 5000, 5000 I started sailing across an ocean.
I did not hit land again until 38000, 38000.
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Apparently this might be a bug in 1.8:
As of 1.8 if you go outside of generation of a 1.7 world an endless ocean generates over regular 1.8 land and you may see underwater trees occasionally.
This seems to happen to newly created worlds in 1.8.
Considering the biome code is unfinished at the moment, it is unknown how intentional the "endless oceans" are. "
I should add that the paragraph above was taken from the Minecraft Wiki.
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According to the Wiki , in 1.1 the Ocean biome can be over 25,000 blocks wide and 30 blocks deep, with the occasional island and the rare mushroom biome.
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After a couple unsuccessful attempts to find myself in a nice looking spawn point on a newly generated map, I decided to find one by myself. I made myself a boat and started my sail due north-east.  Three hours later, 15.000 meters further, I'm still on sail, looking not only for a decent land, but ANY land at all.  How vast are oceans in Minecraft 1.9 pre-release 3? Can I still hope for some highlands far away from my original spawn point?
How vast are Minecraft's oceans?
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The size of a single ocean is the maximum size of a biome. However, two oceans can be placed together. The size of the oceans is estimated between 25.000-30.000 blocks or could be any size if oceans combine.
The size of the oceans is estimated between 25.000-30.000 blocks or could be any size if oceans combine.