What is the room size of 216 sq feet?

The question cannot be answered because the area of a room does not provide sufficient information.

The room could be 1 ft * 216 ft

or 2 ft by 108 ft

or 3 ft by 72 ft

or 4 ft by 54 ft

etc.

And that is only dealing with integer values. The length and breadth could be fractions or irrational.