The possible forms of comparison would be:
1) Carla es más alta que Cristina. (Carla is taller than Cristina.)
This type of comparison follows the next order:
verb ser + más + adjective + que.
2) If we want to be precise, we could add the exact information and say:
Carla es 5 centímetros más alta que Cristina.
3) We could also say that 'Cristina is shorter than Carla.'
Cristina es más baja que Carla.
We use the same construction, but we just change the adjective alta (tall) for baja (short).
4) And finally, in Spanish, we can say that 'Cristina is less tall than Carla.'
Cristina es menos alta que Carla.
In this case, we just apply the adverb menos meaning less.