Iron, Cobalt and Nickel possess nearly same size. Why?

ferrous cobalt and nickel are having atomic numbers 26,27,28 respectively. so they are arranged side by side in the periodic table. if elements has nearly same atomic number then they would have nearly same size.

because of that three are the metals.

They have nearly same size as they are in the same period which means that they have same no. of shells. Moreover they come next to each other i.e 26,27,28 which makes the no. of electrons in valence shell almost same. so they have same size