Suppose we are building a pure iron-56 planet atom by atom, how large can it get in terms of radius before it collapses into a black hole?
It won't ever collapse into a black hole, it will collapse to form a neutron star.
As you add mass to your planet, initially it will get larger, perhaps till it reaches a few times the size of the Earth, when it is a fraction of a Jupiter mass. After that, it will get smaller because it's structure is governed by electron degeneracy.
It will shrink until it has a radius of about 2000 km, when it is just over a solar mass. Beyond that it is unstable and will collapse. There will be a large explosion, but the remnant is not massive enough to avoid forming a stable neutron star.
More details here https://physics.stackexchange.com/q/196520/123208
The only way you might do it is to add the mass very quickly so that the iron star becomes very hot and not supported by degeneracy pressure. It would need to be hotter than about $5\times 10^8$ K, just like the centre of a massive star. Then, if it were more massive than about 2.5 solar masses and you allowed it to cool, it might collapse to form a black hole.