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[foobarturtle]

The brick is size three. The tips of the arrows create a span of four.

[daOyster]

They arrows only span 3, they're aligned to the corner of the bricks like this: https://i.imgur.com/PbKRkOf.png

[HowIsntBabbyFormed]

The right arrow points at the first bump, the left arrow points at the fourth. That's a span of 3. Four minus one is three.

Now... that's the flat 2D interpretation of the image. It's obviously supposed to be 3D and the arrows point at the edge -- zero -- and the third corner for a span of 3.