the size is more or less confirmed already, both by devs and by players in the workshop. the exact shape of the hitbox is not yet confirmed, but its believed to be a sphere.
i did a workshop just now where i crossed rein shields in a squareshape and trapped an arrow fired by hanzo within those squares by reducing projectile speed and gravity to 0 (making the fired arrow stand still mid-air). i wasnt really able to check the exact size cause im not picky enough to spend the time to do things properly, but it seems to be around 0.2m as some others have observed and it is very clear that the arrow hitbox is not as big as it looks.
i made a video showing the arrow sitting in air without hitting the barriers even though the arrow clearly looks long enough to hit them. the arrow hitbox is at the back of the arrow and not at the front as you would expect.
the front of the arrow is only visual and the arrow hitbox is smaller than the “cage” created by the barriers.
https://streamable.com/2zv9s