Apparently the new 1.7.2 update to Minecraft introduced this change among others:
Portals to the Nether can now have different shapes
Can anyone elaborate this? What shapes? What part of their construction has changed, and what remains the same?
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The portal must be rectangular (e.g. 3x3, 4x2, 3x5, but no circles).
It has a maximum internal size of 21x21.
It must be at least 3 tall, and at least 2 wide (not including the frame, with the frame that ends up as 5x4).
If you build multiple portals in front of each other and light them, only the rear most portal will be visible, but all will function. This may be a bug.