Rooms of impossible size are rooms whose interior dimensions are not what should be physically allowed based on their outside dimension. They may be larger, smaller, or have impossible layouts given the constraints of their outside structure. This may be due to physics warping technology, magic, supernatural involvement, or the clever use of a portal or teleportation gate. They may be found in wizard towers, schools of magic, haunted houses, laboratories of mad science, and elsewhere.

The strange dimensions of a room may be obvious from first sight, or it may take characters some time (and perhaps some investigation) to determine that something is not right.