Atomic Size Trends:

* Across a Period (Left to Right): Atomic size generally decreases. This is because the number of protons increases, attracting the electrons more strongly and pulling them closer to the nucleus.

* Down a Group (Top to Bottom): Atomic size generally increases. This is because the number of electron shells increases, adding to the atomic radius.

Cobalt (Co) is located in period 4, group 9 of the periodic table.

Meitnerium (Mt) is located in period 7, group 9 of the periodic table.

Since Meitnerium is located in a lower period than Cobalt, it has more electron shells, resulting in a larger atomic radius.