The perfect party

As you may recall, the mesklenits set out on an expedition. Once, the leader needed to send a special squad for reconnaissance, consisting of the best mesklenits. To do this, he lined up the whole team in a row.

The color of each mesklenit's shell is represented by an uppercase Latin letter ($A$ to $Z$). To save time, the leader decided to pick several consecutive members from the row. Additionally, he considered that reconnaissance would be more successful if the selected squad is symmetric with respect to their shell colors. For example, the squad $\text{RGBGR}$ is symmetric, whereas $\text{RGRB}$ is not.

You need to pick the largest possible squad of mesklenits from the row that satisfies these conditions.