Hypotenuse
Hypotenuse – The longest side of a right angle triangle, opposite the right angle.

Our high pot (hypotenuse) is the longest one.
Or

This high pot in use (hypotenuse) for the longest length.
Example 1

In this right angled triangle the hypotenuse is the longest side.
Example 2

The hypotenuse of this triangle is 15cm long.