Why are the atomic radii of Cr, Mn, Fe, and Co almost the same?

Because they all have electrons in the same shells (up to the 4s and 3d orbitals), which are generally at similar distances from the nuclei (this is the definition of the radius); the reason they AREN'T actually identical is that the larger atoms have higher positive charges on their nuclei that bring the electrons in a bit closer. For atoms as big as these, a difference of +1, +2, or even +3 isn't that significant in terms of how much closer the electrons will be pulled.

Now, if it was between hydrogen and helium, you would be DOUBLING the charge of the nucleus when you go from +1 to +2. With chromium and manganese, you are only increasing the atomic charge by ~4% when you go from +24 to +25. The change gets more and more subtle the farther down you go in the periodic table!

Atomic Radius Of Manganese