Put Steve in a 1x1 hole and measure the wiggle room. ALL the room - WiggleRoom = Steve
In a 1x1 hole, I pointed the camera straight down and moved from corner to corner. Regardless of corner or orientation, the crosshairs pointed at the farthest point of a 5x5 square from the corner.
16 = 5 + 6 + 5
That's 16 total square width, 5 for the each side of Steve, and 6 for the wiggle room.
Front/back/left/right, the closest you can get to a wall is 5. This test is unable to confirm if Steve is 10x10 square, or a 5 radius circle.
Note: this finding contradicts the original question, as Steve is 0.625 wide, and the asker was expecting more than that. However the original question also supports this answer in the door screenshot (5 texture pixels from door).
It seems reasonable to think that Steve is an unrotating 0.625x0.625 square, which is 0.88 from corner to corner, and could not fit through the ~0.7 gap.
Here is Steve (in green), blocked by two stairs (in purple). Steve is 10 units big, but the yellow line is ~14.14 units big, which is too large to go through the ~11.3 unit opening.
![Steve trapped](https://i.stack.imgur.com/XjJMt.jpg)
This is caused by the spawning algorithm. On the ocean, there are significantly less areas suitable for spawning. When you build a house over/on the ocean, you provide a large suitable area for them to spawn in, and because it is the only one in the area, everything that would normally be spread out around there instead spawns inside your house.
Best Answer
Unfortunately i don't think there is a way out without following the corridor back the way you came, I'm still trying to figure this one out.
If you're really desperate though you could try opening the game to LAN with cheats enabled, typing
/gamerule keepInventory true
followed by/kill
to try respawning at the spawn point along with all your items (the gamerule command stops you losing your items on death).Unfortunately these worlds have a habit of changing your spawn point when you're inside the other dimension so you might be a touch screwed. I seem to remember spawning in some obsidian and building a portal to the nether though, Once i was inside i was able to build another portal and go through which generated an exit on the overworld (regular world) and i was able to escape. Only catch was i had to spawn in the actual Portal item that makes up the purple stuff on the inside
/give playername 90
Give those a shot see how you get on :)